diff --git a/cypress/e2e/basic.cy.js b/cypress/e2e/basic.cy.js
index b1b856119..5a0a767ed 100644
--- a/cypress/e2e/basic.cy.js
+++ b/cypress/e2e/basic.cy.js
@@ -52,12 +52,25 @@ describe('After setup instance', () => {
 		cy.intercept('POST', '/api/signup').as('signup');
 
 		cy.get('[data-cy-signup]').click();
+		cy.get('[data-cy-signup-submit]').should('be.disabled');
 		cy.get('[data-cy-signup-username] input').type('alice');
+		cy.get('[data-cy-signup-submit]').should('be.disabled');
 		cy.get('[data-cy-signup-password] input').type('alice1234');
+		cy.get('[data-cy-signup-submit]').should('be.disabled');
 		cy.get('[data-cy-signup-password-retype] input').type('alice1234');
+		cy.get('[data-cy-signup-submit]').should('not.be.disabled');
 		cy.get('[data-cy-signup-submit]').click();
 
 		cy.wait('@signup');
+
+		cy.visitHome();
+
+		// ユーザー名が重複している場合の挙動確認
+		cy.get('[data-cy-signup]').click();
+		cy.get('[data-cy-signup-username] input').type('alice');
+		cy.get('[data-cy-signup-password] input').type('alice1234');
+		cy.get('[data-cy-signup-password-retype] input').type('alice1234');
+		cy.get('[data-cy-signup-submit]').should('be.disabled');
   });
 });
 
diff --git a/packages/frontend/src/components/MkSignup.vue b/packages/frontend/src/components/MkSignup.vue
index 62ada6b73..fa6948139 100644
--- a/packages/frontend/src/components/MkSignup.vue
+++ b/packages/frontend/src/components/MkSignup.vue
@@ -117,7 +117,9 @@ const shouldDisableSubmitting = $computed((): boolean => {
 		instance.enableHcaptcha && !hCaptchaResponse ||
 		instance.enableRecaptcha && !reCaptchaResponse ||
 		instance.enableTurnstile && !turnstileResponse ||
-		passwordRetypeState === 'not-match';
+		instance.emailRequiredForSignup && emailState !== 'ok' ||
+		usernameState !== 'ok' ||
+		passwordRetypeState !== 'match';
 });
 
 function onChangeUsername(): void {