Microfossil data around Kuroko deposits of the Shakanai mine are re-examined, based on the recent development of fossil datum levels. It is indicated through the examination on the list of planktonic spieces among foraminiferan fossils described by OTAGAKI (1966) that the lower and middle horizons of M₂ mudstone overlying Kuroko deposits are correlated to BLOW'S (1969) N9. Nannofossils described in m³ mudstone by SASAKI and HIRAYAMA (1983) can be correlated to CN3 and CN4, not in accordance with their conclusion. The recent works on fossil datum levels assigned the boundary between CN3 and CN4, to 16.2 Ma in age and that between N8 and N9 to 15.2 Ma, respectively. Furthermore, the occurrence of planktonic foraminifera correlated to N7 has not been described from the relevant strata in the inner side of northeastern Japan arc.
It is concluded that m³ and M₂ mudstone beds deposited during periods between 16.5 and 15.5 Ma and between 15.2 and 14.7 Ma, respectively. Therefore, it is indicated that the Kuroko deposits in the Shakanai mine which occur beneath the M₂ mudstone bed, were formed about 15.2 Ma ago. The age is correlated to the middle Nishikurosawa in the Cenozoic standard stratigraphy of the Northeast Japan.