Paleomagnetic measurements have been made on tuff and mud samples from 25 sites of the Morozaki Group and the Mizunami Group, distributed in the eastern part of Southwest Japan. Stability tests by alternating field and thermal demagnetization reveal the existence of characteristic remanent magnetization, of which directions show significant clockwise deflection; the mean declination shift exceeds 50° in the late Early Miocene members (N6-N8), and is less than 30° in the early Middle Miocene members (N9). This result, compared with the other paleomagnetic data from the Setouchi Miocene Series and the volcanic rocks in the San'in district, suggests that coherent rotation of Southwest Japan began between the late Early Miocene and the early Middle Miocene. Nearly a half of the rotation might have been attained in the stage of Zone N9, which possibly corresponds to the period of the opening and increasing subsidence in the Japan Sea.