diff --git a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
index 7185830..b5937f1 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
@@ -95,7 +95,7 @@
 
     public void deleteAllBooks(String accountId)
     {
-        booksMap.get(accountId).clear();
+        booksMap.remove(accountId);
     }
 
     //（（（　目標・振り返りは省略　）））