Nagoya Gen'i(1628-1696) is known as a pioneer of the Koho-ha School of Japanese Kampo. This research examined his medical system and established that his medical theory was based on the following five features : (1) Medical Philosophy, (2) Pathology, (3) Theory of Formula Construction, (4) Theory of the Effects of Individual Herbal Medicines and (5) Attitudes toward Treatment. Gen'i authored numerous books on features 1-4. He emphasized the medical theories in 1, Medical Philosophy, but numerous contradictions were found upon a comparison of the medical works he authored. People of the Gosei-ha School at that time selected a basic formula that suited them from among a variety of formulas and tried to organize their own medical systems on its basis. While Gen'i argued against this stance late in his life, he tried to organized a medical system by combining the five features with the medical philosophy and the formula theory from "Shanghanlun ". Medical studies from his later years are noted as the forefront of the Koho-ha School of Japanese Kampo.