diff --git a/tool/mk_builtin_loader.rb b/tool/mk_builtin_loader.rb
index 9d4b6405a1..fbed9fa335 100644
--- a/tool/mk_builtin_loader.rb
+++ b/tool/mk_builtin_loader.rb
@@ -212,7 +212,11 @@ def mk_builtin_header file
     f.puts 'struct rb_execution_context_struct; /* in vm_core.h */'
     f.puts
     lineno = 11
-    line_file = file.gsub('\\', '/')
+    line_file = file
+    if File::ALT_SEPARATOR
+      line_file = line_file.tr(File::ALT_SEPARATOR, File::SEPARATOR)
+      ofile = ofile.tr(File::ALT_SEPARATOR, File::SEPARATOR)
+    end
 
     inlines.each{|cfunc_name, (body_lineno, text, params, func_name)|
       if String === cfunc_name