The distribution of benthic foraminifers is controlled by such a parameter as salinity and more strikingly by temperature, both of which define water masses. By compiling the distribution of Recent foraminifers reported by previous workers, I have distinguished five species-groups among the modern foraminifers inhabiting shelf seas around the Japanese Islands. The Species-group I consists of tropical species. Its northern limit is located off the Boso Peninsula (35°N) in the Pacific and off the Tsugaru Peninsula (41.5°N) in the Japan Sea. These locations coincide with the position of 17℃ isotherm of the annual maximum temperature at 100 m water depth on both sides of the Honshu Island, reflecting the distribution of two warm water bodies, the Kuroshio and Tsushima Currents, on two sides of the Japanese Islands. The distributional limit of the Species-group II (subtropical species) extends much further northwards, coinciding with 10℃ isotherm of maximum temperature. The distributional pattern of this group results from a remarkable invasion of warm waters into cooler seas in the form of Tsugaru and Soya Warm Currents, both of which are in reality northward extensional currents of the Tsushima Warm Current. The Species-group III (temperate-water species) also characterizes most the shallow seas around the Japanese Islands, except for those waters warmer than 20℃ isotherm of maximum temperature. The species-groups IV (subarctic species) and V (arctic ones) occur in region dominated by the Oyashio Current, including the Pacific, northern part of the Japan Sea, and the Okhotsk Sea. The southern distributional limits of IV and V groups coincide with 9℃ and 5℃ isotherms of maximum temperature, respectively. Around the Honshu Island, constituent species of both groups are found only in the bathyal zone. These relationships between the foraminiferal species and water masses are considered to provide a database with which Cenozoic oceanographic conditions, particularly relative thermal structures of surface waters around Japan can be reconstructed.