diff --git a/__test__/index.spec.ts b/__test__/index.spec.ts
index a8d2945..dfcee35 100644
--- a/__test__/index.spec.ts
+++ b/__test__/index.spec.ts
@@ -204,6 +204,89 @@ test('POST: 400', async () => {
   ).rejects.toHaveProperty('response.status', 400)
 })
 
+test('POST: nested validation', async () => {
+  const res1 = await client.users.post({
+    body: {
+      id: 123,
+      name: 'foo',
+      location: {
+        country: 'JP',
+        stateProvince: 'Tokyo'
+      }
+    }
+  })
+  expect(res1.status).toBe(204)
+
+  // Note that extraneous properties are allowed by default
+  const res2 = await client.users.post({
+    body: {
+      id: 123,
+      name: 'foo',
+      location: {
+        country: 'JP',
+        stateProvince: 'Tokyo',
+        extra1: {
+          extra1a: 'bar',
+          extra1b: 'baz'
+        }
+      },
+      extra2: 'qux'
+    } as any
+  })
+  expect(res2.status).toBe(204)
+})
+
+test('POST: 400 (nested validation)', async () => {
+  // id is not a number
+  await expect(
+    client.users.post({
+      body: {
+        id: '123',
+        name: 'foo',
+        location: {
+          country: 'JP',
+          stateProvince: 'Tokyo'
+        }
+      } as any
+    })
+  ).rejects.toHaveProperty('response.status', 400)
+
+  // location is missing
+  await expect(
+    client.users.post({
+      body: { id: 123, name: 'foo' } as any
+    })
+  ).rejects.toHaveProperty('response.status', 400)
+
+  // country is not a valid 2-letter country code
+  await expect(
+    client.users.post({
+      body: {
+        id: 123,
+        name: 'foo',
+        location: {
+          country: 'XX',
+          stateProvince: 'Tokyo'
+        }
+      } as any
+    })
+  ).rejects.toHaveProperty('response.status', 400)
+
+  // stateProvince is not a string
+  await expect(
+    client.users.post({
+      body: {
+        id: 123,
+        name: 'foo',
+        location: {
+          country: 'JP',
+          stateProvince: 1234
+        }
+      } as any
+    })
+  ).rejects.toHaveProperty('response.status', 400)
+})
+
 test('controller dependency injection', async () => {
   let val = 0
   const id = '5'
diff --git a/package.json b/package.json
index 364e808..7938177 100644
--- a/package.json
+++ b/package.json
@@ -95,6 +95,7 @@
     "@typescript-eslint/eslint-plugin": "^4.28.1",
     "@typescript-eslint/parser": "^4.28.1",
     "axios": "^0.21.1",
+    "class-transformer": "^0.5.1",
     "class-validator": "^0.13.1",
     "eslint": "^7.30.0",
     "eslint-config-prettier": "^8.3.0",
@@ -109,6 +110,7 @@
     "jest": "^27.0.6",
     "node-fetch": "^2.6.1",
     "prettier": "^2.3.2",
+    "reflect-metadata": "^0.1.13",
     "rimraf": "^3.0.2",
     "standard-version": "^9.3.0",
     "ts-jest": "^27.0.3",
diff --git a/servers/all/$server.ts b/servers/all/$server.ts
index 88abc85..e8cc420 100644
--- a/servers/all/$server.ts
+++ b/servers/all/$server.ts
@@ -1,9 +1,13 @@
 /* eslint-disable */
 // prettier-ignore
-import multipart, { FastifyMultipartAttactFieldsToBodyOptions, Multipart } from 'fastify-multipart'
+import 'reflect-metadata'
+// prettier-ignore
+import { ClassTransformOptions, plainToInstance } from 'class-transformer'
 // prettier-ignore
 import { validateOrReject, ValidatorOptions } from 'class-validator'
 // prettier-ignore
+import multipart, { FastifyMultipartAttactFieldsToBodyOptions, Multipart } from 'fastify-multipart'
+// prettier-ignore
 import * as Validators from './validators'
 // prettier-ignore
 import hooksFn0 from './api/hooks'
@@ -43,6 +47,7 @@ import type { FastifyInstance, RouteHandlerMethod, preValidationHookHandler, Fas
 // prettier-ignore
 export type FrourioOptions = {
   basePath?: string
+  transformer?: ClassTransformOptions
   validator?: ValidatorOptions
   multipart?: FastifyMultipartAttactFieldsToBodyOptions
 }
@@ -260,6 +265,7 @@ const asyncMethodToHandler = (
 // prettier-ignore
 export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
   const basePath = options.basePath ?? ''
+  const transformerOptions: ClassTransformOptions = { enableCircularCheck: true, ...options.transformer }
   const validatorOptions: ValidatorOptions = { validationError: { target: false }, ...options.validator }
   const hooks0 = hooksFn0(fastify)
   const hooks1 = hooksFn1(fastify)
@@ -292,7 +298,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
         callParserIfExistsQuery(parseBooleanTypeQueryParams([['bool', false, false], ['optionalBool', true, false], ['boolArray', false, true], ['optionalBoolArray', true, true]])),
         normalizeQuery,
         createValidateHandler(req => [
-          Object.keys(req.query as any).length ? validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions) : null
+          Object.keys(req.query as any).length ? validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions) : null
         ])
       ]
     },
@@ -310,8 +316,8 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
         formatMultipartData([]),
         normalizeQuery,
         createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions),
-          validateOrReject(Object.assign(new Validators.Body(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions),
+          validateOrReject(plainToInstance(Validators.Body, req.body as any, transformerOptions), validatorOptions)
         ])
       ]
     },
@@ -344,7 +350,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
       preValidation: [
         formatMultipartData([['requiredArr', false], ['optionalArr', true], ['empty', true], ['vals', false], ['files', false]]),
         createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.MultiForm(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.MultiForm, req.body as any, transformerOptions), validatorOptions)
         ])
       ]
     },
@@ -404,7 +410,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
       onRequest: [...hooks0.onRequest, hooks2.onRequest],
       preParsing: hooks0.preParsing,
       preValidation: createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.UserInfo(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.UserInfo, req.body as any, transformerOptions), validatorOptions)
         ]),
       preHandler: ctrlHooks1.preHandler
     } as RouteShorthandOptions,
diff --git a/servers/all/api/users/_userId@number/controller.ts b/servers/all/api/users/_userId@number/controller.ts
index c6acbb2..5889807 100644
--- a/servers/all/api/users/_userId@number/controller.ts
+++ b/servers/all/api/users/_userId@number/controller.ts
@@ -5,5 +5,15 @@ export type AdditionalRequest = {
 }
 
 export default defineController(() => ({
-  get: ({ params }) => ({ status: 200, body: { id: params.userId, name: 'bbb' } })
+  get: ({ params }) => ({
+    status: 200,
+    body: {
+      id: params.userId,
+      name: 'bbb',
+      location: {
+        country: 'JP',
+        stateProvince: 'Tokyo'
+      }
+    }
+  })
 }))
diff --git a/servers/all/api/users/controller.ts b/servers/all/api/users/controller.ts
index 31aef00..040c344 100644
--- a/servers/all/api/users/controller.ts
+++ b/servers/all/api/users/controller.ts
@@ -16,6 +16,18 @@ const hooks = defineHooks(() => ({
 export { hooks, AdditionalRequest }
 
 export default defineController(() => ({
-  get: async () => ({ status: 200, body: [{ id: 1, name: 'aa' }] }),
+  get: async () => ({
+    status: 200,
+    body: [
+      {
+        id: 1,
+        name: 'aa',
+        location: {
+          country: 'JP',
+          stateProvince: 'Tokyo'
+        }
+      }
+    ]
+  }),
   post: () => ({ status: 204 })
 }))
diff --git a/servers/all/validators/index.ts b/servers/all/validators/index.ts
index 2397f34..68e5116 100644
--- a/servers/all/validators/index.ts
+++ b/servers/all/validators/index.ts
@@ -1,3 +1,4 @@
+import { Type } from 'class-transformer'
 import {
   IsNumberString,
   IsBooleanString,
@@ -8,7 +9,10 @@ import {
   IsString,
   Allow,
   IsOptional,
-  ArrayNotEmpty
+  ArrayNotEmpty,
+  IsISO31661Alpha2,
+  ValidateNested,
+  IsObject
 } from 'class-validator'
 import type { ReadStream } from 'fs'
 
@@ -52,12 +56,27 @@ export class Body {
   file: File | ReadStream
 }
 
+export class UserInfoLocation {
+  @IsISO31661Alpha2()
+  country: string
+
+  @IsString()
+  stateProvince: string
+}
+
 export class UserInfo {
   @IsInt()
   id: number
 
   @MaxLength(20)
   name: string
+
+  // @Type decorator is required to validate nested object properly
+  // @IsObject decorator is required or class-validator will not throw an error when the property is missing
+  @ValidateNested()
+  @IsObject()
+  @Type(() => UserInfoLocation)
+  location: UserInfoLocation
 }
 
 export class MultiForm {
diff --git a/servers/noMulter/$server.ts b/servers/noMulter/$server.ts
index a0e2bd9..ccfb92c 100644
--- a/servers/noMulter/$server.ts
+++ b/servers/noMulter/$server.ts
@@ -1,5 +1,9 @@
 /* eslint-disable */
 // prettier-ignore
+import 'reflect-metadata'
+// prettier-ignore
+import { ClassTransformOptions, plainToInstance } from 'class-transformer'
+// prettier-ignore
 import { validateOrReject, ValidatorOptions } from 'class-validator'
 // prettier-ignore
 import * as Validators from './validators'
@@ -27,6 +31,7 @@ import type { FastifyInstance, RouteHandlerMethod, preValidationHookHandler, Fas
 // prettier-ignore
 export type FrourioOptions = {
   basePath?: string
+  transformer?: ClassTransformOptions
   validator?: ValidatorOptions
 }
 
@@ -122,6 +127,7 @@ const asyncMethodToHandler = (
 // prettier-ignore
 export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
   const basePath = options.basePath ?? ''
+  const transformerOptions: ClassTransformOptions = { enableCircularCheck: true, ...options.transformer }
   const validatorOptions: ValidatorOptions = { validationError: { target: false }, ...options.validator }
   const hooks0 = hooksFn0(fastify)
   const hooks1 = hooksFn1(fastify)
@@ -139,7 +145,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
     {
       onRequest: [hooks0.onRequest, ctrlHooks0.onRequest],
       preValidation: createValidateHandler(req => [
-          Object.keys(req.query as any).length ? validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions) : null
+          Object.keys(req.query as any).length ? validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions) : null
         ])
     },
     asyncMethodToHandler(controller0.get)
@@ -150,8 +156,8 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
     {
       onRequest: [hooks0.onRequest, ctrlHooks0.onRequest],
       preValidation: createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions),
-          validateOrReject(Object.assign(new Validators.Body(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions),
+          validateOrReject(plainToInstance(Validators.Body, req.body as any, transformerOptions), validatorOptions)
         ])
     },
     methodToHandler(controller0.post)
@@ -203,7 +209,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
     {
       onRequest: [hooks0.onRequest, hooks1.onRequest],
       preValidation: createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.UserInfo(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.UserInfo, req.body as any, transformerOptions), validatorOptions)
         ]),
       preHandler: ctrlHooks1.preHandler
     } as RouteShorthandOptions,
diff --git a/servers/noTypedParams/$server.ts b/servers/noTypedParams/$server.ts
index 0e1a3f9..f3eb808 100644
--- a/servers/noTypedParams/$server.ts
+++ b/servers/noTypedParams/$server.ts
@@ -1,9 +1,13 @@
 /* eslint-disable */
 // prettier-ignore
-import multipart, { FastifyMultipartAttactFieldsToBodyOptions, Multipart } from 'fastify-multipart'
+import 'reflect-metadata'
+// prettier-ignore
+import { ClassTransformOptions, plainToInstance } from 'class-transformer'
 // prettier-ignore
 import { validateOrReject, ValidatorOptions } from 'class-validator'
 // prettier-ignore
+import multipart, { FastifyMultipartAttactFieldsToBodyOptions, Multipart } from 'fastify-multipart'
+// prettier-ignore
 import * as Validators from './validators'
 // prettier-ignore
 import hooksFn0 from './api/hooks'
@@ -31,6 +35,7 @@ import type { FastifyInstance, RouteHandlerMethod, preValidationHookHandler, Fas
 // prettier-ignore
 export type FrourioOptions = {
   basePath?: string
+  transformer?: ClassTransformOptions
   validator?: ValidatorOptions
   multipart?: FastifyMultipartAttactFieldsToBodyOptions
 }
@@ -146,6 +151,7 @@ const asyncMethodToHandler = (
 // prettier-ignore
 export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
   const basePath = options.basePath ?? ''
+  const transformerOptions: ClassTransformOptions = { enableCircularCheck: true, ...options.transformer }
   const validatorOptions: ValidatorOptions = { validationError: { target: false }, ...options.validator }
   const hooks0 = hooksFn0(fastify)
   const hooks1 = hooksFn1(fastify)
@@ -165,7 +171,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
     {
       onRequest: [hooks0.onRequest, ctrlHooks0.onRequest],
       preValidation: createValidateHandler(req => [
-          Object.keys(req.query as any).length ? validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions) : null
+          Object.keys(req.query as any).length ? validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions) : null
         ])
     },
     asyncMethodToHandler(controller0.get)
@@ -178,8 +184,8 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
       preValidation: [
         formatMultipartData([]),
         createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.Query(), req.query as any), validatorOptions),
-          validateOrReject(Object.assign(new Validators.Body(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.Query, req.query as any, transformerOptions), validatorOptions),
+          validateOrReject(plainToInstance(Validators.Body, req.body as any, transformerOptions), validatorOptions)
         ])
       ]
     },
@@ -201,7 +207,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
       preValidation: [
         formatMultipartData([['empty', false], ['vals', false], ['files', false]]),
         createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.MultiForm(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.MultiForm, req.body as any, transformerOptions), validatorOptions)
         ])
       ]
     },
@@ -246,7 +252,7 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
     {
       onRequest: [hooks0.onRequest, hooks1.onRequest],
       preValidation: createValidateHandler(req => [
-          validateOrReject(Object.assign(new Validators.UserInfo(), req.body as any), validatorOptions)
+          validateOrReject(plainToInstance(Validators.UserInfo, req.body as any, transformerOptions), validatorOptions)
         ]),
       preHandler: ctrlHooks1.preHandler
     } as RouteShorthandOptions,
diff --git a/src/buildServerFile.ts b/src/buildServerFile.ts
index f638d0c..d358d29 100644
--- a/src/buildServerFile.ts
+++ b/src/buildServerFile.ts
@@ -29,10 +29,16 @@ export default (input: string, project?: string) => {
 
   return {
     text: addPrettierIgnore(`/* eslint-disable */${
+      hasValidator
+        ? "\nimport 'reflect-metadata'" +
+          "\nimport { ClassTransformOptions, plainToInstance } from 'class-transformer'" +
+          "\nimport { validateOrReject, ValidatorOptions } from 'class-validator'"
+        : ''
+    }${
       hasMultipart
         ? "\nimport multipart, { FastifyMultipartAttactFieldsToBodyOptions, Multipart } from 'fastify-multipart'"
         : ''
-    }${hasValidator ? "\nimport { validateOrReject, ValidatorOptions } from 'class-validator'" : ''}
+    }
 ${hasValidator ? "import * as Validators from './validators'\n" : ''}${imports}${
       hasMultipart ? "import type { ReadStream } from 'fs'\n" : ''
     }import type { LowerHttpMethod, AspidaMethods, HttpStatusOk, AspidaMethodParams } from 'aspida'
@@ -46,7 +52,7 @@ import type { FastifyInstance, RouteHandlerMethod${
 
 export type FrourioOptions = {
   basePath?: string
-${hasValidator ? '  validator?: ValidatorOptions\n' : ''}${
+${hasValidator ? '  transformer?: ClassTransformOptions\n  validator?: ValidatorOptions\n' : ''}${
       hasMultipart ? '  multipart?: FastifyMultipartAttactFieldsToBodyOptions\n' : ''
     }}
 
@@ -262,7 +268,8 @@ export default (fastify: FastifyInstance, options: FrourioOptions = {}) => {
   const basePath = options.basePath ?? ''
 ${
   hasValidator
-    ? '  const validatorOptions: ValidatorOptions = { validationError: { target: false }, ...options.validator }\n'
+    ? '  const transformerOptions: ClassTransformOptions = { enableCircularCheck: true, ...options.transformer }\n' +
+      '  const validatorOptions: ValidatorOptions = { validationError: { target: false }, ...options.validator }\n'
     : ''
 }${consts}
 ${
diff --git a/src/createControllersText.ts b/src/createControllersText.ts
index 2588053..3dae9ef 100644
--- a/src/createControllersText.ts
+++ b/src/createControllersText.ts
@@ -482,9 +482,9 @@ ${validateInfo
     v.type
       ? `          ${
           v.hasQuestion ? `Object.keys(req.${v.name} as any).length ? ` : ''
-        }validateOrReject(Object.assign(new Validators.${checker.typeToString(v.type)}(), req.${
+        }validateOrReject(plainToInstance(Validators.${checker.typeToString(v.type)}, req.${
           v.name
-        } as any), validatorOptions)${v.hasQuestion ? ' : null' : ''}`
+        } as any, transformerOptions), validatorOptions)${v.hasQuestion ? ' : null' : ''}`
       : ''
   )
   .join(',\n')}\n        ])`
diff --git a/yarn.lock b/yarn.lock
index 1c3a25a..a8edb65 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1555,6 +1555,11 @@ cjs-module-lexer@^1.0.0:
   resolved "https://registry.yarnpkg.com/cjs-module-lexer/-/cjs-module-lexer-1.2.1.tgz#2fd46d9906a126965aa541345c499aaa18e8cd73"
   integrity sha512-jVamGdJPDeuQilKhvVn1h3knuMOZzr8QDnpk+M9aMlCaMkTDd6fBWPhiDqFvFZ07pL0liqabAiuy8SY4jGHeaw==
 
+class-transformer@^0.5.1:
+  version "0.5.1"
+  resolved "https://registry.yarnpkg.com/class-transformer/-/class-transformer-0.5.1.tgz#24147d5dffd2a6cea930a3250a677addf96ab336"
+  integrity sha512-SQa1Ws6hUbfC98vKGxZH3KFY0Y1lm5Zm0SY8XX9zbK7FJCyVEac3ATW0RIpwzW+oOfmHE5PMPufDG9hCfoEOMw==
+
 class-validator@^0.13.1:
   version "0.13.1"
   resolved "https://registry.yarnpkg.com/class-validator/-/class-validator-0.13.1.tgz#381b2001ee6b9e05afd133671fbdf760da7dec67"
@@ -4636,6 +4641,11 @@ redent@^3.0.0:
     indent-string "^4.0.0"
     strip-indent "^3.0.0"
 
+reflect-metadata@^0.1.13:
+  version "0.1.13"
+  resolved "https://registry.yarnpkg.com/reflect-metadata/-/reflect-metadata-0.1.13.tgz#67ae3ca57c972a2aa1642b10fe363fe32d49dc08"
+  integrity sha512-Ts1Y/anZELhSsjMcU605fU9RE4Oi3p5ORujwbIKXfWa+0Zxs510Qrmrce5/Jowq3cHSZSJqBjypxmHarc+vEWg==
+
 regexpp@^3.0.0, regexpp@^3.1.0:
   version "3.1.0"
   resolved "https://registry.yarnpkg.com/regexpp/-/regexpp-3.1.0.tgz#206d0ad0a5648cffbdb8ae46438f3dc51c9f78e2"