The Tonosho Group in Teshima Island, Shodo-gun, Kagawa Prefecture comprises three formations ; Teshima, Ieura and Karato formations in ascending order. The Teshima Formation yielded some marine molluscan fossils (SAITO, 1962 and SAITO et al., 1970) that indicate the Middle Miocene in age. This is the first record of an Isognomon species from the Miocene Teshima Formation and is described as new to science. It may be a relict species of the Mesozoic isognomonid fauna.