Discovery of Gaudryceras tombetsense Matsumoto, 1984 in the Futakawa Formation of the Sotoizumi Group in the Aridagawa area, Wakayama, southwestern Japan establishes that this formation includes sediments of early Late Maastrichtian age, which is the youngest record of the Sotoizumi Group distributed in Shikoku and the Kii Peninsula. The occurrence of the youngest fossils from the easternmost area of the Sotoizumi Group may support the suggestion that the depocenter of the Sotoizumi basin migrated eastward due to movement along the Kurosegawa Tectonic Zone (KTZ), which was still active during Campanian to Maastrichtian time.