According to his theory, the distinction of characters observed in Manyo-gana was caused by Toraijin (people from overseas, especially from China and Korea, who settled in early Japan and introduced Continental culture) who had distinguished the difference of the sounds that were not unimportant for Japanese.
すなわち、万葉仮名に見られる用字の使い分けは渡来人が日本語にとって不必要であった音声の違いを音韻として読み取ってしまったものだとするものである。