diff --git a/locales/index.d.ts b/locales/index.d.ts
index 440f24ac84..c4bab73d8a 100644
--- a/locales/index.d.ts
+++ b/locales/index.d.ts
@@ -7152,6 +7152,36 @@ export interface Locale extends ILocale {
          * 入力されたメールアドレス({email})宛に確認のメールが送信されました。メールに記載されたリンクにアクセスすると、アカウントの作成が完了します。メールに記載されているリンクの有効期限は30分です。
          */
         "emailSent": ParameterizedString<"email">;
+        "_errors": {
+            /**
+             * メールアドレスが入力されていないか、不正な値です。
+             */
+            "emailInvalid": string;
+            /**
+             * このメールアドレスを使用して登録することはできません。
+             */
+            "emailNotAllowed": string;
+            /**
+             * 招待コードが入力されていないか、不正な値です。
+             */
+            "invitationCodeInvalid": string;
+            /**
+             * 招待コードが見つからなかったか、既に使用されています。
+             */
+            "invitationCodeNotFoundOrUsed": string;
+            /**
+             * 招待コードの有効期限が切れています。
+             */
+            "invitationCodeExpired": string;
+            /**
+             * このユーザー名は既に使用されています。
+             */
+            "usernameAlreadyUsed": string;
+            /**
+             * このユーザー名で登録することはできません。
+             */
+            "usernameNotAllowed": string;
+        };
     };
     "_accountDelete": {
         /**
diff --git a/locales/ja-JP.yml b/locales/ja-JP.yml
index 5d8e1a5e72..066505bdf6 100644
--- a/locales/ja-JP.yml
+++ b/locales/ja-JP.yml
@@ -1853,6 +1853,14 @@ _signup:
   almostThere: "ほとんど完了です"
   emailAddressInfo: "あなたが使っているメールアドレスを入力してください。メールアドレスが公開されることはありません。"
   emailSent: "入力されたメールアドレス({email})宛に確認のメールが送信されました。メールに記載されたリンクにアクセスすると、アカウントの作成が完了します。メールに記載されているリンクの有効期限は30分です。"
+  _errors:
+    emailInvalid: "メールアドレスが入力されていないか、不正な値です。"
+    emailNotAllowed: "このメールアドレスを使用して登録することはできません。"
+    invitationCodeInvalid: "招待コードが入力されていないか、不正な値です。"
+    invitationCodeNotFoundOrUsed: "招待コードが見つからなかったか、既に使用されています。"
+    invitationCodeExpired: "招待コードの有効期限が切れています。"
+    usernameAlreadyUsed: "このユーザー名は既に使用されています。"
+    usernameNotAllowed: "このユーザー名で登録することはできません。"
 
 _accountDelete:
   accountDelete: "アカウントの削除"
diff --git a/packages/backend/src/core/SignupService.ts b/packages/backend/src/core/SignupService.ts
index 3865392b7f..87fce599b4 100644
--- a/packages/backend/src/core/SignupService.ts
+++ b/packages/backend/src/core/SignupService.ts
@@ -18,6 +18,7 @@ import generateUserToken from '@/misc/generate-native-user-token.js';
 import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { InstanceActorService } from '@/core/InstanceActorService.js';
 import { bindThis } from '@/decorators.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 import UsersChart from '@/core/chart/charts/users.js';
 import { UtilityService } from '@/core/UtilityService.js';
 import { UserService } from '@/core/UserService.js';
@@ -59,13 +60,13 @@ export class SignupService {
 
 		// Validate username
 		if (!this.userEntityService.validateLocalUsername(username)) {
-			throw new Error('INVALID_USERNAME');
+			throw new IdentifiableError('be85f7f4-1dd3-4107-bce4-07cdb0cbb0c3', 'INVALID_USERNAME');
 		}
 
 		if (password != null && passwordHash == null) {
 			// Validate password
 			if (!this.userEntityService.validatePassword(password)) {
-				throw new Error('INVALID_PASSWORD');
+				throw new IdentifiableError('d5f4959c-a881-41e8-b755-718fbf161258', 'INVALID_PASSWORD');
 			}
 
 			// Generate hash of password
@@ -78,12 +79,12 @@ export class SignupService {
 
 		// Check username duplication
 		if (await this.usersRepository.exists({ where: { usernameLower: username.toLowerCase(), host: IsNull() } })) {
-			throw new Error('DUPLICATED_USERNAME');
+			throw new IdentifiableError('d412327a-1bd7-4b70-a982-7eec000db8fc', 'DUPLICATED_USERNAME');
 		}
 
 		// Check deleted username duplication
 		if (await this.usedUsernamesRepository.exists({ where: { username: username.toLowerCase() } })) {
-			throw new Error('USED_USERNAME');
+			throw new IdentifiableError('dd5f52be-2c95-4c39-ba45-dc2d74b3dd81', 'USED_USERNAME');
 		}
 
 		const isTheFirstUser = !await this.instanceActorService.realLocalUsersPresent();
@@ -91,7 +92,7 @@ export class SignupService {
 		if (!opts.ignorePreservedUsernames && !isTheFirstUser) {
 			const isPreserved = this.meta.preservedUsernames.map(x => x.toLowerCase()).includes(username.toLowerCase());
 			if (isPreserved) {
-				throw new Error('USED_USERNAME');
+				throw new IdentifiableError('adad138b-9c63-41bf-931e-6b050fd3bb8d', 'DENIED_USERNAME');
 			}
 		}
 
@@ -121,7 +122,9 @@ export class SignupService {
 				host: IsNull(),
 			});
 
-			if (exist) throw new Error(' the username is already used');
+			if (exist) {
+				throw new IdentifiableError('d412327a-1bd7-4b70-a982-7eec000db8fc', 'DUPLICATED_USERNAME');
+			}
 
 			account = await transactionalEntityManager.save(new MiUser({
 				id: this.idService.gen(),
diff --git a/packages/backend/src/server/api/SignupApiService.ts b/packages/backend/src/server/api/SignupApiService.ts
index 3ec5e5d3e6..ab8cfd1e90 100644
--- a/packages/backend/src/server/api/SignupApiService.ts
+++ b/packages/backend/src/server/api/SignupApiService.ts
@@ -16,6 +16,7 @@ import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { EmailService } from '@/core/EmailService.js';
 import { MiLocalUser } from '@/models/User.js';
 import { FastifyReplyError } from '@/misc/fastify-reply-error.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 import { bindThis } from '@/decorators.js';
 import { L_CHARS, secureRndstr } from '@/misc/secure-rndstr.js';
 import { SigninService } from './SigninService.js';
@@ -74,6 +75,11 @@ export class SignupApiService {
 	) {
 		const body = request.body;
 
+		function error(status: number, error: { id: string, [x: string]: string }) {
+			reply.code(status);
+			return { error };
+		}
+
 		// Verify *Captcha
 		// ただしテスト時はこの機構は障害となるため無効にする
 		if (process.env.NODE_ENV !== 'test') {
@@ -115,24 +121,30 @@ export class SignupApiService {
 		const emailAddress = body['emailAddress'];
 
 		if (this.meta.emailRequiredForSignup) {
-			if (emailAddress == null || typeof emailAddress !== 'string') {
-				reply.code(400);
-				return;
+			if (emailAddress == null || typeof emailAddress !== 'string' || emailAddress === '') {
+				return error(400, {
+					id: '33b104c9-2f22-4640-b27a-40979bde4a77',
+					message: 'Email address is not present or is invalid.',
+				});
 			}
 
 			const res = await this.emailService.validateEmailForAccount(emailAddress);
 			if (!res.available) {
-				reply.code(400);
-				return;
+				return error(400, {
+					id: '75ece55a-7869-49b1-b796-c0634224fcae',
+					message: 'You cannot use this email address.',
+				});
 			}
 		}
 
 		let ticket: MiRegistrationTicket | null = null;
 
 		if (this.meta.disableRegistration) {
-			if (invitationCode == null || typeof invitationCode !== 'string') {
-				reply.code(400);
-				return;
+			if (invitationCode == null || typeof invitationCode !== 'string' || invitationCode === '') {
+				return error(400, {
+					id: 'c8324ccf-7153-47a0-90a3-682eb06ba10d',
+					message: 'Invitation code is not present or is invalid.',
+				});
 			}
 
 			ticket = await this.registrationTicketsRepository.findOneBy({
@@ -140,47 +152,69 @@ export class SignupApiService {
 			});
 
 			if (ticket == null || ticket.usedById != null) {
-				reply.code(400);
-				return;
+				return error(400, {
+					id: 'f08118af-8358-441c-b992-b5b0bbd337d2',
+					message: 'Invitation code not found or already used.',
+				});
 			}
 
 			if (ticket.expiresAt && ticket.expiresAt < new Date()) {
-				reply.code(400);
-				return;
+				return error(400, {
+					id: '3277822c-29dd-4bc9-ad57-47af702f78b8',
+					message: 'Invitation code has expired.',
+				});
 			}
 
 			// メアド認証が有効の場合
 			if (this.meta.emailRequiredForSignup) {
 				// メアド認証済みならエラー
 				if (ticket.usedBy) {
-					reply.code(400);
-					return;
+					return error(400, {
+						id: 'f08118af-8358-441c-b992-b5b0bbd337d2',
+						message: 'Invitation code not found or already used.',
+					});
 				}
 
 				// 認証しておらず、メール送信から30分以内ならエラー
 				if (ticket.usedAt && ticket.usedAt.getTime() + (1000 * 60 * 30) > Date.now()) {
-					reply.code(400);
-					return;
+					return error(400, {
+						id: 'f08118af-8358-441c-b992-b5b0bbd337d2',
+						message: 'Invitation code not found or already used.',
+					});
 				}
 			} else if (ticket.usedAt) {
-				reply.code(400);
-				return;
+				return error(400, {
+					id: 'f08118af-8358-441c-b992-b5b0bbd337d2',
+					message: 'Invitation code not found or already used.',
+				});
 			}
 		}
 
 		if (this.meta.emailRequiredForSignup) {
 			if (await this.usersRepository.exists({ where: { usernameLower: username.toLowerCase(), host: IsNull() } })) {
-				throw new FastifyReplyError(400, 'DUPLICATED_USERNAME');
+				return error(400, {
+					id: '9c20a0c3-c9e7-418f-8058-767f4e345bd4',
+					code: 'DUPLICATED_USERNAME',
+					message: 'Username already exists.',
+				});
 			}
 
 			// Check deleted username duplication
 			if (await this.usedUsernamesRepository.exists({ where: { username: username.toLowerCase() } })) {
-				throw new FastifyReplyError(400, 'USED_USERNAME');
+				return error(400, {
+					id: '90e84f35-599a-468c-b420-98139fe9f988',
+					code: 'USED_USERNAME',
+					message: 'Username was previously used by another user.',
+				});
 			}
 
 			const isPreserved = this.meta.preservedUsernames.map(x => x.toLowerCase()).includes(username.toLowerCase());
 			if (isPreserved) {
-				throw new FastifyReplyError(400, 'DENIED_USERNAME');
+				return error(400, {
+					id: 'e26cbcc3-7a0c-4cf4-988f-533f56ca72bf',
+					code: 'DENIED_USERNAME',
+					message: 'This username is not allowed.',
+				});
 			}
 
 			const code = secureRndstr(16, { chars: L_CHARS });
@@ -236,6 +270,41 @@ export class SignupApiService {
 					token: secret,
 				};
 			} catch (err) {
+				if (err instanceof IdentifiableError) {
+					switch (err.id) {
+						case 'be85f7f4-1dd3-4107-bce4-07cdb0cbb0c3':
+							return error(400, {
+								id: 'f6bff66c-a3f9-48b8-b56c-3c3ffc49bfdd',
+								code: 'INVALID_USERNAME',
+								message: 'Username is invalid.',
+							});
+						case 'd5f4959c-a881-41e8-b755-718fbf161258':
+							return error(400, {
+								id: '6dffa54e-9f5f-4c07-9662-e5c75ab63ee5',
+								code: 'INVALID_PASSWORD',
+								message: 'Password is invalid.',
+							});
+						case 'd412327a-1bd7-4b70-a982-7eec000db8fc':
+							return error(400, {
+								id: '9c20a0c3-c9e7-418f-8058-767f4e345bd4',
+								code: 'DUPLICATED_USERNAME',
+								message: 'Username already exists.',
+							});
+						case 'dd5f52be-2c95-4c39-ba45-dc2d74b3dd81':
+							return error(400, {
+								id: '90e84f35-599a-468c-b420-98139fe9f988',
+								code: 'USED_USERNAME',
+								message: 'Username was previously used by another user.',
+							});
+						case 'adad138b-9c63-41bf-931e-6b050fd3bb8d':
+							return error(400, {
+								id: 'e26cbcc3-7a0c-4cf4-988f-533f56ca72bf',
+								code: 'DENIED_USERNAME',
+								message: 'This username is not allowed.',
+							});
+					}
+				}
+
 				throw new FastifyReplyError(400, typeof err === 'string' ? err : (err as Error).toString());
 			}
 		}
@@ -247,11 +316,20 @@ export class SignupApiService {
 
 		const code = body['code'];
 
+		function error(status: number, error: { id: string, [x: string]: string }) {
+			reply.code(status);
+			return { error };
+		}
+
 		try {
 			const pendingUser = await this.userPendingsRepository.findOneByOrFail({ code });
 
 			if (this.idService.parse(pendingUser.id).date.getTime() + (1000 * 60 * 30) < Date.now()) {
-				throw new FastifyReplyError(400, 'EXPIRED');
+				return error(400, {
+					id: 'e8b5b1ce-c7fe-456f-b06b-467cd16c060f',
+					code: 'EXPIRED',
+					message: 'This link has expired.',
+				});
 			}
 
 			const { account, secret } = await this.signupService.signup({
diff --git a/packages/frontend/src/components/MkSignupDialog.form.vue b/packages/frontend/src/components/MkSignupDialog.form.vue
index 3d1c44fc90..176ed024e3 100644
--- a/packages/frontend/src/components/MkSignupDialog.form.vue
+++ b/packages/frontend/src/components/MkSignupDialog.form.vue
@@ -268,6 +268,7 @@ async function onSubmit(): Promise<void> {
 
 	const res = await fetch(`${config.apiUrl}/signup`, {
 		method: 'POST',
+		redirect: 'error',
 		headers: {
 			'Content-Type': 'application/json',
 		},
@@ -278,14 +279,14 @@ async function onSubmit(): Promise<void> {
 	});
 
 	if (res) {
-		if (res.status === 204 || instance.emailRequiredForSignup) {
+		if (res.status === 204 && instance.emailRequiredForSignup) {
 			os.alert({
 				type: 'success',
 				title: i18n.ts._signup.almostThere,
 				text: i18n.tsx._signup.emailSent({ email: email.value }),
 			});
 			emit('signupEmailPending');
-		} else {
+		} else if (res.status === 200) {
 			const resJson = (await res.json()) as Misskey.entities.SignupResponse;
 			if (_DEV_) console.log(resJson);
 
@@ -294,23 +295,67 @@ async function onSubmit(): Promise<void> {
 			if (props.autoSet) {
 				await login(resJson.token);
 			}
+		} else {
+			const resJson = (await res.json()) as {
+				error?: {
+					id: string;
+					code?: string;
+					message?: string;
+				};
+			};
+
+			let message: string | null = null;
+
+			if (resJson.error != null) {
+				if (resJson.error.message != null) {
+					message = resJson.error.message;
+				} else if (resJson.error.id != null) {
+					message = i18n.ts.somethingHappened + '\n' + resJson.error.id;
+				}
+
+				switch (resJson.error.id) {
+					case '33b104c9-2f22-4640-b27a-40979bde4a77':
+						message = i18n.ts._signup._errors.emailInvalid;
+						break;
+					case '75ece55a-7869-49b1-b796-c0634224fcae':
+						message = i18n.ts._signup._errors.emailNotAllowed;
+						break;
+					case 'c8324ccf-7153-47a0-90a3-682eb06ba10d':
+						message = i18n.ts._signup._errors.invitationCodeInvalid;
+						break;
+					case '3277822c-29dd-4bc9-ad57-47af702f78b8':
+						message = i18n.ts._signup._errors.invitationCodeExpired;
+						break;
+					case 'f08118af-8358-441c-b992-b5b0bbd337d2':
+						message = i18n.ts._signup._errors.invitationCodeNotFoundOrUsed;
+						break;
+					case '9c20a0c3-c9e7-418f-8058-767f4e345bd4':
+					case '90e84f35-599a-468c-b420-98139fe9f988':
+						message = i18n.ts._signup._errors.usernameAlreadyUsed;
+						break;
+					case 'e26cbcc3-7a0c-4cf4-988f-533f56ca72bf':
+						message = i18n.ts._signup._errors.usernameNotAllowed;
+						break;
+				}
+			}
+
+			onSignupApiError(message ? { title: i18n.ts.somethingHappened, text: message } : undefined);
 		}
 	}
 
 	submitting.value = false;
 }
 
-function onSignupApiError() {
+function onSignupApiError(message?: { title?: string; text: string }): void {
 	submitting.value = false;
 	hcaptcha.value?.reset?.();
 	mcaptcha.value?.reset?.();
 	recaptcha.value?.reset?.();
 	turnstile.value?.reset?.();
 	testcaptcha.value?.reset?.();
-
 	os.alert({
 		type: 'error',
-		text: i18n.ts.somethingHappened,
+		...(message ?? { text: i18n.ts.somethingHappened }),
 	});
 }
 </script>