Kumagusu also made important contributions to the study of Japanese folklore.
His study of folklore had two main characteristics. The first was that he conducted comparative research of folklore in international contexts, and the second was that he took relatively unusual topics like sexology as his object of study as well.
He published research notes related to folklore in Nature and Notes and Queries, and wrote countless articles for Japanese academic journals.
Kumagusu exchanged many letters to Yanagita Kunio, “the father of folklore,” and Yanagita praised Kumagusu by calling him “the utmost boundary of the potential of the Japanese.”