diff --git a/src/main/java/org/ntlab/citrusserver/resources/AccountsRest.java b/src/main/java/org/ntlab/citrusserver/resources/AccountsRest.java
index ed7b686..1a430e6 100644
--- a/src/main/java/org/ntlab/citrusserver/resources/AccountsRest.java
+++ b/src/main/java/org/ntlab/citrusserver/resources/AccountsRest.java
@@ -47,6 +47,10 @@
     public String signup(@FormParam("account_id") String accountId, @FormParam("password") String password) {
         String token;
         token = accountManager.createAccount(accountId, password);
+        if (token == null){
+            var response = Response.status(Response.Status.CONFLICT).entity("id '" + accountId +  "' は既に存在します");//404
+            throw new WebApplicationException(response.build());
+        }
         return token;
     }