diff --git a/packages/backend/src/server/api/common/signup.ts b/packages/backend/src/server/api/common/signup.ts
index f246a454e..3ac4ac0ba 100644
--- a/packages/backend/src/server/api/common/signup.ts
+++ b/packages/backend/src/server/api/common/signup.ts
@@ -10,6 +10,7 @@ import { UserKeypair } from '@/models/entities/user-keypair.js';
 import { usersChart } from '@/services/chart/index.js';
 import { UsedUsername } from '@/models/entities/used-username.js';
 import { db } from '@/db/postgre.js';
+import { ApiError } from '@/server/api/error.js';
 import generateUserToken from './generate-native-user-token.js';
 
 export async function signup(opts: {
@@ -23,13 +24,25 @@ export async function signup(opts: {
 
 	// Validate username
 	if (!Users.validateLocalUsername(username)) {
-		throw new Error('INVALID_USERNAME');
+		throw new ApiError({
+			message: 'This username is invalid.',
+			code: 'INVALID_USERNAME',
+			id: 'ece89f3c-d845-4d9a-850b-1735285e8cd4',
+			kind: 'client',
+			httpStatusCode: 400,
+		});
 	}
 
 	if (password != null && passwordHash == null) {
 		// Validate password
 		if (!Users.validatePassword(password)) {
-			throw new Error('INVALID_PASSWORD');
+			throw new ApiError({
+				message: 'This password is invalid.',
+				code: 'INVALID_PASSWORD',
+				id: 'a941905b-fe7b-43e2-8ecd-50ad3a2287ab',
+				kind: 'client',
+				httpStatusCode: 400,
+			});
 		}
 
 		// Generate hash of password
@@ -40,14 +53,22 @@ export async function signup(opts: {
 	// Generate secret
 	const secret = generateUserToken();
 
+	const duplicateUsernameError = {
+		message: 'This username is not available.',
+		code: 'USED_USERNAME',
+		id: '7ddd595e-6860-4593-93c5-9fdbcb80cd81',
+		kind: 'client',
+		httpStatusCode: 409,
+	};
+
 	// Check username duplication
 	if (await Users.findOneBy({ usernameLower: username.toLowerCase(), host: IsNull() })) {
-		throw new Error('DUPLICATED_USERNAME');
+		throw new ApiError(duplicateUsernameError);
 	}
 
 	// Check deleted username duplication
 	if (await UsedUsernames.findOneBy({ username: username.toLowerCase() })) {
-		throw new Error('USED_USERNAME');
+		throw new ApiError(duplicateUsernameError);
 	}
 
 	const keyPair = await new Promise<string[]>((res, rej) =>
@@ -76,7 +97,7 @@ export async function signup(opts: {
 			host: IsNull(),
 		});
 
-		if (exist) throw new Error(' the username is already used');
+		if (exist) throw new ApiError(duplicateUsernameError);
 
 		account = await transactionalEntityManager.save(new User({
 			id: genId(),