The detailed investigation of stratigraphic distribution of molluscan shells and planktonic foraminifera in the Lower Pleistocene Omma Formation in Kanazawa, Ishikawa Prefecture, Japan, provided the following conclusions : (1) the Tsushima Current flowed into the Japan Sea at 3.2Ma (MIS KM 5 or 3), 2.9Ma (MIS G 17 or 15), and 1.9Ma (MIS 59), in the 3.5-1.7Ma interval ; (2) the altitude of the southern strait of the Japan Sea was about 50m above the present-day sea level during 3.3-2.5Ma ; (3) the early formation of strait was located at southwest of Cheju Island ; (4) the current flowed at every interglacial highstand, except for MIS 25, 23, and 21.3, in the 1.71-0.8Ma interval ; (5) the intermediate water of the Japan Sea (approximately 200m depth) at MIS 47, 45, 43, 41, and 29 was the warmest in the Quaternary, and was gradually followed by coldness. In addition, the stratigraphic data indicated that during the interglacial period, when the average monthly July insolation was greater than 520W/m2 at 65°N latitude, a significant decrease in molluscan species diversity and abundance occurred along the coast of the Sea of Japan in Honshu, Japan. It is highly likely that this temporal condition was caused by an increase in seasonal differences caused by a faster rise in summer temperatures than winter temperatures.