In this study, we carried out zircon U-Pb dating of greenish tuff deposits from the eastern base of Yotei Volcano, southwest Hokkaido, Japan, to investigate the Miocene ignimbrite volcanism associated with the opening of the Japan Sea. This tuff may be correlated with the Pepenai-gawa green tuff member of the Wakikata Formation. Zircon grains from the tuff yield an eruption age of 5.7±0.2 and 5.6±0.2 Ma based on the youngest five zircon grains from a total of twenty analyses and using a weighted mean value and the Tera-Wasserburg method, respectively. The tuff also includes zircon grains with ages of 8-25 Ma. These zircons may represent xenocrysts that formed in the crust beneath Hokkaido due to intermittent heat flux during opening of the Japan Sea.