diff --git a/server.go b/server.go
index 888b9ae..74b4f94 100644
--- a/server.go
+++ b/server.go
@@ -3,6 +3,7 @@ package overflow_gateway_websocket
 import (
 	"log"
 	"net/http"
+	"sync"
 
 	"git.loafle.net/overflow/overflow_gateway_websocket/websocket"
 	"github.com/valyala/fasthttp"
@@ -20,6 +21,7 @@ type server struct {
 	_upgrader *websocket.Upgrader
 	_handlers map[string]*ClientOptions
 	_clients  map[string]Client
+	_cMtx     sync.Mutex
 }
 
 func NewServer(o *ServerOptions) Server {
@@ -53,25 +55,26 @@ func (s *server) onDisconnected(c Client) {
 
 func (s *server) onConnection(ctx *fasthttp.RequestCtx) {
 	path := string(ctx.Path())
+	co, ok := s._handlers[path]
+	if !ok {
+		ctx.Response.Header.Set("Sec-Websocket-Version", "13")
+		ctx.Error(http.StatusText(fasthttp.StatusNotFound), fasthttp.StatusNotFound)
+
+		log.Printf("Path[%s] is not exist.", path)
+		return
+	}
 
 	s._upgrader.Upgrade(ctx, nil, func(conn *websocket.Conn, err error) {
 		if err != nil {
 			log.Print("upgrade:", err)
 			return
 		}
-		co, ok := s._handlers[path]
-		if !ok {
-			ctx.Response.Header.Set("Sec-Websocket-Version", "13")
-			ctx.Error(http.StatusText(fasthttp.StatusNotFound), fasthttp.StatusNotFound)
-
-			log.Printf("Path[%s] is not exist.", path)
-			conn.Close()
-			return
-		}
-
+		s._cMtx.Lock()
 		cid := s._option.IDGenerator(ctx)
 		c := NewClient(cid, path, co, conn)
 		s._clients[cid] = c
+		s._cMtx.Unlock()
+
 		s._option.OnConnection(path, c)
 	})
 }