diff --git a/packages/backend/src/core/EmailService.ts b/packages/backend/src/core/EmailService.ts
index 95a1290125fd..c2c74a426024 100644
--- a/packages/backend/src/core/EmailService.ts
+++ b/packages/backend/src/core/EmailService.ts
@@ -166,6 +166,13 @@ export class EmailService {
 		available: boolean;
 		reason: null | 'used' | 'format' | 'disposable' | 'mx' | 'smtp' | 'banned' | 'network' | 'blacklist' | 'allowedOnly';
 	}> {
+		if (this.utilityService.validateEmail(emailAddress)) {
+			return {
+				available: false,
+				reason: 'format',
+			};
+		}
+		
 		const exist = await this.userProfilesRepository.countBy({
 			emailVerified: true,
 			email: emailAddress,
diff --git a/packages/backend/src/core/UtilityService.ts b/packages/backend/src/core/UtilityService.ts
index 3974e69ae503..f89dc1b6fc1b 100644
--- a/packages/backend/src/core/UtilityService.ts
+++ b/packages/backend/src/core/UtilityService.ts
@@ -39,6 +39,14 @@ export class UtilityService {
 		return this.punyHost(uri) === this.toPuny(this.config.host);
 	}
 
+	// メールアドレスのバリデーションを行う
+	// https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address
+	@bindThis
+	public validateEmail(email: string): boolean {
+		const regexp = /^[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/;
+		return regexp.test(email);
+	}
+
 	@bindThis
 	public isBlockedHost(blockedHosts: string[], host: string | null): boolean {
 		if (host == null) return false;