We investigated the biostratigraphy of the Miocene Kumi Formation at the type locality on Oki Dogo Island, Shimane Prefecture, using planktonic foraminifera, calcareous nannofossils and diatoms. The Kumi Formation is divided into upper and lower parts. The upper part is mainly composed of alternating sandstones and mudstones with associated glauconite key marker beds, and has an exposed thickness of about 100 m. The lower part reaches 90 m in thickness and consists of diatomaceous siltstone. The lower part is correlated with planktonic foraminiferal Zone N.8, calcareous nannofossil Zone CN3-CN4 and diatom Zone NPD3A. The upper part contains an interval assigned to diatom Zone NPD5C. These zonal assignments are concordant with each other, based on the current geologic time scale. With respect to previous studies, two of the glauconite-rich layers in the upper part can be widely correlated in the circum-Japan Sea area.13;