diff --git a/pkg/domain/app_website.go b/pkg/domain/app_website.go
index 31bf3bfcb..2fbb4fb82 100644
--- a/pkg/domain/app_website.go
+++ b/pkg/domain/app_website.go
@@ -151,6 +151,10 @@ func (w *Website) Validate() error {
 	return nil
 }
 
+func (w *Website) Normalize() {
+	w.FQDN = strings.ToLower(w.FQDN)
+}
+
 func (w *Website) pathComponents() []string {
 	// NOTE: empty PathPrefix must not exist
 	if w.PathPrefix == "/" {
diff --git a/pkg/usecase/apiserver/app_service.go b/pkg/usecase/apiserver/app_service.go
index 7d338f146..89b5fbee8 100644
--- a/pkg/usecase/apiserver/app_service.go
+++ b/pkg/usecase/apiserver/app_service.go
@@ -62,6 +62,9 @@ func (s *Service) CreateApplication(ctx context.Context, app *domain.Application
 	}
 	app.OwnerIDs = repo.OwnerIDs
 
+	for _, website := range app.Websites {
+		website.Normalize()
+	}
 	// Validate
 	err = s.validateApp(ctx, app)
 	if err != nil {
@@ -213,6 +216,9 @@ func (s *Service) UpdateApplication(ctx context.Context, id string, args *domain
 	}
 	app.Apply(args)
 
+	for _, website := range app.Websites {
+		website.Normalize()
+	}
 	// Validate
 	if err = s.validateApp(ctx, app); err != nil {
 		return err