diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index c5f7a8f5a..b114b4563 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -36,7 +36,7 @@
 #endif
 
 #ifdef _MSC_VER
-typedef int pid_t;
+typedef mrb_int pid_t;
 #endif
 
 #include <fcntl.h>