diff --git a/tool/transform_mjit_header.rb b/tool/transform_mjit_header.rb
index d844b5a2ac..1565d5fcfd 100644
--- a/tool/transform_mjit_header.rb
+++ b/tool/transform_mjit_header.rb
@@ -9,7 +9,7 @@ PROGRAM = File.basename($0, ".*")
 
 module MJITHeader
   ATTR_VALUE_REGEXP  = /[^()]|\([^()]*\)/
-  ATTR_REGEXP        = /__attribute__\s*\(\((#{ATTR_VALUE_REGEXP})*\)\)/
+  ATTR_REGEXP        = /__attribute__\s*\(\(#{ATTR_VALUE_REGEXP}*\)\)/
   FUNC_HEADER_REGEXP = /\A(\s*#{ATTR_REGEXP})*[^\[{(]*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
   TARGET_NAME_REGEXP = /\A(rb|ruby|vm|insn|attr)_/