Three species of the cirolanid isopod genus Bathynomus are reported from the Miocene deposits of central Japan. Bathynomus kominatoensis sp. nov. and an unidentified Bathynomus sp. 1 are described from the upper Miocene Amatsu Formation in the Kominato-Osawa area, Chiba Prefecture. Another large species, Bathynomus sp. 2, was obtained from the middle Miocene Negishi Formation in the Iwadono Hills area, Saitama Prefecture. The new species most resembles Bathynomus giganteus, inhabiting the deep-sea floor of the Western Atlantic and Caribbean Sea. In addition to the large-sized body, Bathynomus kominatoensis sp. nov. and Bathynomus sp. 2 are included in the supergiant species group of the genus based upon morphology of the pleotelson and uropods. Both occurrences extend the geographic and stratigraphic ranges for the supergiant species group of Bathynomus into the Miocene of central Japan, although no species of the supergiant group is distributed in modern Japanese waters.