Off-Rebun Island, Hokkaido, is listed as the type locality of Phyllophoridae Thyonidium diomedeae (Ohshima, 1915). Two specimens of Thyonidium diomedeae were collected from sand-and-silt bottom of Off-Okayama, Kibiseto in Seto Inland Sea, at the depth of 11m. Occurrence of the species has been reported only from northern Japan and northern Alaska, and this report revises the southern limit of the species distribution.