Organic carbon contents and hydrocarbons of the Pliocene Kuwae Formation along the Tainai River, Kitakanbara area, Niigata Prefecture were investigated to reconstruct the paleoceanography of the Japan Sea. Between 2.5Ma and 3.5Ma, TOC (total organic carbon) contents changed between 0.3% and 1% with a small oscillation (range: 0.3%). In this period, the sea floor was anoxic, and the origin of the organic carbon was mainly marine plankton. The oscillation in TOC seems to correspond to the Milankovich short cycle (20-40ky). The TOC contents were high during warm periods. This could be due to upwelling activity and nutrient supply from land areas. TOC contents also show a long interval change. After 2.8Ma, decreased TOC suggests that primary productivity in the marine system decreased in the cold climate.