Igneous activity in the Shinji Rift System related to opening of the Japan Sea began in the Early Miocene and continued through to the Pleistocene. New K-Ar ages have been determined for a Pleistocene basalt from a borehole at Eshima bridge and a Miocene rhyolite from Awashima, southern Yumigahama Sandbar. The Pleistocene basalt yielded a whole rock K-Ar age of 0.19±0.01 Ma. A biotite age of 17.48±0.40 Ma for the Miocene rhyolite indicates that it belongs to the Kawai Formation. The Eshima borehole shows that the Eshima-Daikonjima basalts were erupted on land, and that their base is now situated 60 meters beneath present-day sea level. This suggests that sea level at 0.19±0.01 Ma was at least 60 meters lower relative to that of the present day.