diff --git a/src/main/java/com/example/cosmos_serversb/jsons/MembersJson.java b/src/main/java/com/example/cosmos_serversb/jsons/MembersJson.java
new file mode 100644
index 0000000..5ef9da5
--- /dev/null
+++ b/src/main/java/com/example/cosmos_serversb/jsons/MembersJson.java
@@ -0,0 +1,31 @@
+package com.example.cosmos_serversb.jsons;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import com.example.cosmos_serversb.entities.*;
+
+
+public class MembersJson {
+    private Set<User> member = new HashSet<>();
+
+    public MembersJson(){
+
+    }
+
+    public MembersJson(Group group){
+        setMember(group.getMembers());
+    }
+
+    public MembersJson(Set<User> set){
+        setMember(set);
+    }
+
+    private void setMember(Set<User> member) {
+        this.member = member;
+    }
+
+    public Set<User> getMember() {
+        return member;
+    }
+}
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..c3be3e5 100644
--- a/src/main/java/com/example/cosmos_serversb/models/Groups.java
+++ b/src/main/java/com/example/cosmos_serversb/models/Groups.java
@@ -9,6 +9,8 @@
 import org.hibernate.Session;
 import org.hibernate.SessionFactory;
 
+import static com.example.cosmos_serversb.models.Users.getUserById;
+
 @Singleton
 public class Groups {
     private static Groups theInstance = null;
@@ -47,6 +49,11 @@
     }
 
     public static Group createGroup(String name, String uId){
+        User user = getUserById(uId);
+        if(user == null){
+            return null; //存在しないユーザがグループを作成しようとした。
+        }
+
         //先にGidとuriの発行を行う
         String gId = getInstance().createGId();
         String uri = baseURI + AppName + "/groups/" + gId;
@@ -78,7 +85,13 @@
         return DeleteResult.SUCCESS;
     }
 
-    public static HashMap<String, String> getMembersListByGid(String gId){
+    public static MembersJson getMembersListByGid(String gId){
+        Session session = SessionManager.getInstance().getSession();
+        Group editGroup = session.get(Group.class, gId);
+        if(editGroup != null){
+            MembersJson members = new MembersJson(editGroup);
+            return members;
+        }
         return null;
     }