diff --git a/handler/message_created.go b/handler/message_created.go
index 4bc3404..6dbe9db 100644
--- a/handler/message_created.go
+++ b/handler/message_created.go
@@ -147,6 +147,22 @@ func (h *BotHandler) invite(p *payload.MessageCreated) {
 			return
 		}
 
+		inOrg, err := h.githubClient.CheckUserInOrg(ctx, gitHubID)
+		if err != nil {
+			log.Println("failed to check user in org: ", err)
+			return
+		}
+
+		if inOrg {
+			_, err := h.traqClient.PostMessage(ctx, p.Message.ChannelID,
+				fmt.Sprintf("GitHubユーザー %s は既に %s に所属しています", gitHubID, h.githubClient.OrgName()))
+			if err != nil {
+				log.Println("failed to post message: ", err)
+			}
+
+			return
+		}
+
 		traQIDs = append(traQIDs, traQID)
 		gitHubIDs = append(gitHubIDs, gitHubID)
 	}
diff --git a/handler/message_created_test.go b/handler/message_created_test.go
index 43844d4..6ab4709 100644
--- a/handler/message_created_test.go
+++ b/handler/message_created_test.go
@@ -26,6 +26,7 @@ func TestInvite(t *testing.T) {
 		messageID        string
 		embedded         []payload.EmbeddedInfo
 		gitHubUserExist  bool
+		belongToOrg      bool
 		postTextFunc     func(test) string
 		postToBotChannel bool
 		invitations      []*model.Invitation
@@ -122,6 +123,18 @@ https://q.trap.jp/messages/%s`, t.messageID)
 				return "GitHubユーザー no-user は存在しません"
 			},
 		},
+		"すでに所属している": {
+			plainText: "@BOT_traP-jp /invite @ikura-hamu ikura-hamu",
+			messageID: messageID,
+			embedded: []payload.EmbeddedInfo{
+				{Type: "user", Raw: "@BOT_traP-jp", ID: botUserID},
+			},
+			gitHubUserExist: true,
+			belongToOrg:     true,
+			postTextFunc: func(test) string {
+				return fmt.Sprintf("GitHubユーザー ikura-hamu は既に traP-jp に所属しています")
+			},
+		},
 	}
 
 	for name, test := range testCases {
@@ -148,6 +161,12 @@ https://q.trap.jp/messages/%s`, t.messageID)
 				CheckUserExistFunc: func(ctx context.Context, userID string) (bool, error) {
 					return test.gitHubUserExist, nil
 				},
+				CheckUserInOrgFunc: func(ctx context.Context, userID string) (bool, error) {
+					return test.belongToOrg, nil
+				},
+				OrgNameFunc: func() string {
+					return "traP-jp"
+				},
 			}
 
 			bh := &BotHandler{
diff --git a/service/github.go b/service/github.go
index 7fc9eda..c5fac72 100644
--- a/service/github.go
+++ b/service/github.go
@@ -11,5 +11,6 @@ import (
 type GitHub interface {
 	SendInvitations(ctx context.Context, invitations []*model.Invitation) error
 	CheckUserExist(ctx context.Context, userID string) (bool, error)
+	CheckUserInOrg(ctx context.Context, userID string) (bool, error)
 	OrgName() string
 }
diff --git a/service/impl/github.go b/service/impl/github.go
index 2e018a6..439d3ab 100644
--- a/service/impl/github.go
+++ b/service/impl/github.go
@@ -97,6 +97,19 @@ func (g *GitHub) CheckUserExist(ctx context.Context, userID string) (bool, error
 	return true, nil
 }
 
+func (g *GitHub) CheckUserInOrg(ctx context.Context, userID string) (bool, error) {
+	_, _, err := g.cl.Organizations.GetOrgMembership(ctx, userID, g.orgName)
+	var gitHubErr *github.ErrorResponse
+	if errors.As(err, &gitHubErr) && gitHubErr.Response.StatusCode == 404 {
+		return false, nil
+	}
+	if err != nil {
+		return false, fmt.Errorf("failed to get GitHub org membership: %w", err)
+	}
+
+	return true, nil
+}
+
 func (g *GitHub) OrgName() string {
 	return g.orgName
 }