Climatological character of the mixed layer in the northwestern part of the subtropical North Pacific and the formation area of Subtropical Mode Water (STMW) are investigated using all available hydrographic data compiled by the Japan Oceanographic Data Center. Various monthly mean properties of the mixed layer averaged over 2° × 5° (lat. × lon.) rectangles are presented. In the heating season, the mixed layer is less thick throughout the region, while in the cooling season, a prominent thick mixed layer whose properties correspond with those of STMW evolves just off the Kuroshio both south and east of Japan. This probable formation area of STMW is further described by presenting scatter diagrams for the set of the individual mixed layers and constructing mean fields of the mixed layer near the Kuroshio, using the thermocline depth as an offshore coordinate. The relationship between properties of the STMW formation area and those of STMW observed in the 137E section (Suga et al., 1989) suggests a plausible Kuroshio recirculation system.