The Shūi tontoku 酬医頓得 is a medical book passed down for generations in a family of physicians in Nagoya, thought to have been compiled by Tashiro Sanki 田代三喜 (1465–1544). The book was substantially influenced by Buddhism and it gives information on how Buddhism or monks were involved in medicine in the Muromachi period. Buddhist influences in the book are seen at several points. First, it describes the Medicine Buddha, known as Bhaiṣajyaguru, as the origin of medical treatment. Second, it explains the relation between the five solid organs and the five essential elements and expounds on the structure of human body and the cause of illness from the viewpoint of Buddhism, based on traditional Chinese medicine theory. Further, the book points out that the ultimate goal of medical practice is to attain complete enlightenment because medicine was originally granted by the Buddha. This paper reviews the content of the book and discusses the Buddha as a guardian for medicine. According to the book, those who practice medicine reflecting on their own mind without relying on superficial knowledge can reach the state of enlightenment that cannot be explained by words, and meet the Buddha for treatment. The book indicates that the Buddha gives humans various medicines and, moreover, these medicines are symbolically allocated in the imaginary world created by the Buddha. This paper reveals that the Buddha described in the Shūi tontoku is the guardian for medical treatment, and the being who leads to enlightenment.