diff --git a/back/src/users/dto/user.dto.ts b/back/src/users/dto/user.dto.ts
index 9d56847..b450afb 100644
--- a/back/src/users/dto/user.dto.ts
+++ b/back/src/users/dto/user.dto.ts
@@ -1,4 +1,4 @@
-import { IsNotEmpty, IsPositive, IsOptional, IsEmail, NotContains, MaxLength } from 'class-validator'
+import { IsNotEmpty, IsPositive, IsOptional, IsEmail, NotContains, MaxLength, IsAlphanumeric } from 'class-validator'
 
 import { ApiProperty } from '@nestjs/swagger'
 import { Express } from 'express'
@@ -10,6 +10,7 @@ export class UserDto {
 
   @IsNotEmpty()
   @NotContains(' ')
+  @IsAlphanumeric()
   @MaxLength(15)
   readonly username: string
 
diff --git a/back/src/users/entity/user.entity.ts b/back/src/users/entity/user.entity.ts
index ca9eef6..f2148bd 100644
--- a/back/src/users/entity/user.entity.ts
+++ b/back/src/users/entity/user.entity.ts
@@ -19,7 +19,7 @@ export class User {
   @Column({ unique: true })
     ftId: number
 
-  @Column({ unique: true, nullable: true })
+  @Column({ nullable: true })
     email: string
 
   @Column({ select: false, nullable: true })
diff --git a/back/src/users/users.controller.ts b/back/src/users/users.controller.ts
index 22ef818..87ec09b 100644
--- a/back/src/users/users.controller.ts
+++ b/back/src/users/users.controller.ts
@@ -211,6 +211,11 @@ export class UsersController {
   ): Promise<User> {
     const user = await this.usersService.findUser(+profile.id)
     if (user == null) throw new BadRequestException('User not found.')
+    if (payload.username !== undefined) {
+      const user2: User | null = await this.usersService.findUserByName(payload.username).catch(() => null)
+      const user2ftId = user2?.ftId
+      if (user2 !== null && user2ftId !== +profile.id) throw new BadRequestException('Username already taken.')
+    }
     await this.usersService.update(user, payload)
     return user
   }