diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 0eb007504..c5f7a8f5a 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -35,6 +35,10 @@
   #include <unistd.h>
 #endif
 
+#ifdef _MSC_VER
+typedef int pid_t;
+#endif
+
 #include <fcntl.h>
 
 #include <errno.h>