He was specialized in studies of medieval Japanese literature and Kinsei Bungaku (Japanese Edo period literature), and rose to the top of his profession by a method that used plenty of historical materials analytically especially in the field of Noh and by exceptional originality.
中世、近世文学研究を専門とし、とくに能楽研究の分野では膨大な史料を分析的に使う手法と卓抜した創見で斯界に一時代を画した。