Translation for webform error message
By default, Drupal (and Webform) show the error message as Field_name is required, so if you want to show an error message that uses something other than field_name, you will have to override/write your own validation function.

The Webform Validation module may be of assistance here, particularly the code examples for custom validation hooks.

P.S. "本文メッセージ" is probably a mistranslation; it most likely should be flipped ("メッセージ本文", "message body").