The strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) of the shell of Crenomytilus grayanus (Dunker) from the Yamanouchi Member of the Akeyo Formation, Mizunami Group in Toki-cho, Mizunami City, Gifu Prefecture, Japan is determined. The strontium age suggests that the Crenomytilus grayanus-bearing horizon has been deposited at 17.8 ± 0.3 Ma. Therefore, the occurrence of C. grayanus in central Japan may have been related to the Early Miocene global cooling event (Mi1b) at 17.8 Ma.