diff --git a/src/main/java/org/ntlab/citrusserver/resources/BooksRest.java b/src/main/java/org/ntlab/citrusserver/resources/BooksRest.java
index 8816ffa..efd8968 100644
--- a/src/main/java/org/ntlab/citrusserver/resources/BooksRest.java
+++ b/src/main/java/org/ntlab/citrusserver/resources/BooksRest.java
@@ -154,7 +154,7 @@
 
         accountCheck(account_id);
         bookManager.registerFavoriteCount(account_id, book_id);
-        return "success";
+        return "\"success\"";
     }
 
     @Path("/{account_id}/books/{book_id}/unregisterFavoriteCount")
@@ -164,7 +164,7 @@
 
         accountCheck(account_id);
         bookManager.unregisterFavoriteCount(account_id, book_id);
-        return "success";
+        return "\"success\"";
     }
 
     ///---------------------------------------------------------------------