diff --git a/include/ruby/win32.h b/include/ruby/win32.h
index 68b8db73e64938..a7c9db81cd025e 100644
--- a/include/ruby/win32.h
+++ b/include/ruby/win32.h
@@ -233,6 +233,7 @@ struct ifaddrs {
 #define IFF_POINTOPOINT IFF_POINTTOPOINT
 #endif
 
+extern void   rb_w32_sysinit(int *, char ***);
 extern DWORD  rb_w32_osid(void);
 extern rb_pid_t  rb_w32_pipe_exec(const char *, const char *, int, int *, int *);
 extern int    flock(int fd, int oper);
diff --git a/ruby.c b/ruby.c
index 5dfa252fe3a76c..318b914b4ad793 100644
--- a/ruby.c
+++ b/ruby.c
@@ -2256,7 +2256,6 @@ void
 ruby_sysinit(int *argc, char ***argv)
 {
 #if defined(_WIN32)
-    void rb_w32_sysinit(int *argc, char ***argv);
     rb_w32_sysinit(argc, argv);
 #endif
     origarg.argc = *argc;