This paper deals with the frame of Japanese premodern farmhouses and considers which roof frame they adopted, post-framing or sasu (similar to principal rafter) framing. Analyzing the transitional forms of the roof frame between post-framing and sasu-framing, the process of transformation of the framework is clarified including the frame with ridge-supporting posts which was undifferentiated roof frame from main frame. It's revealed that post-framing systems were superior in the western Japan, and that the several forms consisted of both a ridge-supporting post and slanting members such as sasu or nobori-bari were appeared in the eastern Japan in the beginning of premodern times.
