The ostracod fauna from the middle Pleistocene Naganuma Formation (ca. 0.5 Ma) in the Sagami Group near Tokyo Bay, central Japan, was investigated. This study reports for the first time an ostracod fauna around 0.5 Ma in the marine oxygen isotope stage (MIS) 13 of Japan. This fauna consists of 58 species, and three assemblages were defined by Q-mode cluster analysis. The depositional environment of each assemblage can be defined as (1) outer bay, influenced by the open sea with relatively high salinity, (2) inner bay, with relatively low salinity, and (3) central bay, with intermediate salinity level between those of (1) and (2). A decrease in water depth occurred from the outer bay through the central bay to the inner bay during the depositional period of the studied horizons. The three most abundant species of each assemblage are Krithe japonica, Bicornucythere bisanensis and Amphileberis nipponica. The fossil fauna does not include the shallow-bay species Neomonoceratina delicata, currently found south of the Japanese main islands. This result is consistent with previous palaeobiogeographical studies of Japanese bay-dwelling ostracods. Its absence near Tokyo Bay at 0.5 Ma would be due to the distance from its original southern habitats and timing of the species' northward migration.