diff --git a/src/main/java/org/ntlab/citrusserver/repositories/FavoriteManager.java b/src/main/java/org/ntlab/citrusserver/repositories/FavoriteManager.java
index b798bfc..3649989 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/FavoriteManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/FavoriteManager.java
@@ -27,7 +27,7 @@
     //取得したotherAccountIdのいいねした本を返す
     public HashSet<Integer> getFavoritesByID(String accountId, String otherAccountId) {
         if(accountManager.getAccount(accountId) == null) return null;
-        if(!favoritesMap.get(accountId).keySet().contains(otherAccountId)) return null;
+        if(!favoritesMap.get(accountId).containsKey(otherAccountId)) return null;
         return favoritesMap.get(accountId).get(otherAccountId);
     }