diff --git a/tool/mk_builtin_binary.rb b/tool/mk_builtin_binary.rb
index a5c962d5c1..eaa560fa26 100644
--- a/tool/mk_builtin_binary.rb
+++ b/tool/mk_builtin_binary.rb
@@ -17,6 +17,13 @@ RubyVM::each_builtin{|feature, iseq|
 
 $stdout = open('builtin_binary.inc', 'wb')
 
+puts <<H
+// -*- c -*-
+// DO NOT MODIFY THIS FILE DIRECTLY.
+// auto-generated file by #{File.basename(__FILE__)}
+
+H
+
 ary.each{|feature, iseq|
   puts "static const unsigned char #{feature}_bin[] = {"
     dump_bin(iseq)
diff --git a/tool/mk_builtin_loader.rb b/tool/mk_builtin_loader.rb
index aa8e46d5c5..cea4d1f99b 100644
--- a/tool/mk_builtin_loader.rb
+++ b/tool/mk_builtin_loader.rb
@@ -36,6 +36,7 @@ def mk_builtin_header file
   collect_builtin(RubyVM::InstructionSequence.compile_file(file, false).to_a, bs = {})
 
   open(ofile, 'w'){|f|
+    f.puts "// -*- c -*-"
     f.puts "// DO NOT MODIFY THIS FILE DIRECTLY."
     f.puts "// auto-generated file"
     f.puts "//   by #{__FILE__}"