Abstract: New rodent fossils found in the Oligo-Miocene sediments in Kyushu, West Japan, are identified to the following four taxa: l) Castoridae gen. et sp. indet. A; 2) Steneofiber sp. from the Late Oligocene Sasebo Group; 3) Castoridae gen. et sp. indet. B; 4) Diatonys shantungensis from the Early to Middle Miocene Nojima Group. Studies of two specimens identified to the family Castoridae gen. et sp. indet. A and B indicate that they are large beavers with the skull of nearly same size as Trogontherium that holds enamel of longitudinally grooved incisors. Resemblance in size and features of the incisor enamel indicates that these specimens could be of the same genus. Furthermore, it is known that Steneofiber is a cosmopolitan genus, however, it has not been found in the Late Oligocene in East Asia. The occurrence of Diatonys shantungensis, on the other hand, strongly suggests that the late Early Miocene mammals from Japan and the East China• belong to the same province in East Asian margin.