diff --git a/mrbgems/hal-win-socket/src/socket_hal.c b/mrbgems/hal-win-socket/src/socket_hal.c
index 7dd7e586b..cfd467b34 100644
--- a/mrbgems/hal-win-socket/src/socket_hal.c
+++ b/mrbgems/hal-win-socket/src/socket_hal.c
@@ -14,7 +14,9 @@
 #include <mruby/presym.h>
 #include "socket_hal.h"
 
+#ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x0501
+#endif
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <windows.h>