diff --git a/src/main/java/com/example/cosmos_serversb/entities/Group.java b/src/main/java/com/example/cosmos_serversb/entities/Group.java
index c900ac4..eb988d8 100644
--- a/src/main/java/com/example/cosmos_serversb/entities/Group.java
+++ b/src/main/java/com/example/cosmos_serversb/entities/Group.java
@@ -78,6 +78,10 @@
         return members;
     }
 
+    public void clearAllMembersForDeleteGroup(){
+        members.clear();
+    }
+
     public void addMember(String uId){
         User user = Users.getInstance().getUserById(uId);
         if(user != null) {
@@ -91,5 +95,12 @@
         }
     }
 
+    public boolean equals(Group group) {
+        if(this.gId == group.getgId()){
+            return true;
+        }
+        return false;
+    }
+
 }
 
diff --git a/src/main/java/com/example/cosmos_serversb/entities/User.java b/src/main/java/com/example/cosmos_serversb/entities/User.java
index e4f663a..592d2ea 100644
--- a/src/main/java/com/example/cosmos_serversb/entities/User.java
+++ b/src/main/java/com/example/cosmos_serversb/entities/User.java
@@ -129,4 +129,15 @@
     public void addGroups(Group group){
         groups.add(group);
     }
+
+    public void exitGroup(Group group){
+        this.groups.remove(group);
+    }
+
+    public boolean equals(User user) {
+        if(this.uId == user.getuId()){
+            return true;
+        }
+        return false;
+    }
 }
\ No newline at end of file
diff --git a/src/main/java/com/example/cosmos_serversb/models/Groups.java b/src/main/java/com/example/cosmos_serversb/models/Groups.java
index 178e40b..0b57aa5 100644
--- a/src/main/java/com/example/cosmos_serversb/models/Groups.java
+++ b/src/main/java/com/example/cosmos_serversb/models/Groups.java
@@ -1,5 +1,6 @@
 package com.example.cosmos_serversb.models;
 
+import java.lang.reflect.Member;
 import java.util.*;
 import javax.inject.Singleton;
 import java.util.Date;
@@ -71,10 +72,18 @@
     }
 
     public static DeleteResult deleteGroup(String gId){
-        Group editGroup = Groups.getInstance().getGroupByGid(gId);
+        Session session = SessionManager.getInstance().getSession();
+        Group editGroup = session.get(Group.class, gId);
         if(editGroup == null){
             return DeleteResult.FAILURE;
         }
+        //削除するグループに所属しているユーザーに、退会してもらう。
+        for(User member : editGroup.getMembers()){
+            member.exitGroup(editGroup);
+        }
+        //削除するグループが持っているユーザの情報を全て削除する。
+        editGroup.clearAllMembersForDeleteGroup();
+        session.delete(editGroup);
         return DeleteResult.SUCCESS;
     }
 
diff --git a/src/main/resources/Group.hbm.xml b/src/main/resources/Group.hbm.xml
index 41104b5..061392e 100644
--- a/src/main/resources/Group.hbm.xml
+++ b/src/main/resources/Group.hbm.xml
@@ -25,12 +25,11 @@
              table = "GROUP_MEMBER"
              lazy = "true"
              inverse = "true"
-             cascade = "all">
+             cascade = "persist, merge">
             <key column="GID"/>
             <many-to-many
                     class="User"
-                    column="UID"
-                    outer-join="auto">
+                    column="UID">
             </many-to-many>
         </set>
     </class>
diff --git a/src/main/resources/User.hbm.xml b/src/main/resources/User.hbm.xml
index de6de5f..d4528e7 100644
--- a/src/main/resources/User.hbm.xml
+++ b/src/main/resources/User.hbm.xml
@@ -22,13 +22,11 @@
         <set name = "groups"
              table = "GROUP_MEMBER"
              lazy = "true"
-             inverse = "false"
-             cascade = "all">
+             cascade = "refresh">
             <key column="UID"/>
             <many-to-many
                     class="Group"
-                    column="GID"
-                    outer-join="auto">
+                    column="GID">
             </many-to-many>
         </set>
     </class>