diff --git a/spec/ruby/optional/capi/ext/thread_spec.c b/spec/ruby/optional/capi/ext/thread_spec.c
index bab5fcc211..80e9392488 100644
--- a/spec/ruby/optional/capi/ext/thread_spec.c
+++ b/spec/ruby/optional/capi/ext/thread_spec.c
@@ -143,6 +143,7 @@ static VALUE thread_spec_ruby_native_thread_p(VALUE self) {
   }
 }
 
+#ifndef _WIN32
 static VALUE false_result = Qfalse;
 static VALUE true_result = Qtrue;
 
@@ -153,6 +154,7 @@ static void *new_thread_check(void *args) {
     return &false_result;
   }
 }
+#endif
 
 static VALUE thread_spec_ruby_native_thread_p_new_thread(VALUE self) {
 #ifndef _WIN32