The Middle Pleistocene Tahara Formation of the Atsumi Group is distriduted in central to eastern Atsumi Peninsula，Aichi Prefecture，central Japan．It is composed of four members：Ikobe Gravel，Akasawa Silt，Kanbe Gravel，and Toshima Sand and Gravel Members．We investigated temporal and spacial distribution of fossil ostracode assemblages and sedimentary facies in the Akasawa Silt Member．This member consists mainly of gravel-rich sandy silt，massive silt to clay containing molluscan fossils，and bioturbated micaceous sandy silt，in ascending order．A laminated clay layer is distinct in the lower part of the member only in central to eastern areas，suggesting anoxic environments．Sixteen ostracode species belonging to ten genera were recognized from 39 samples of 87．All extant today are in recent brackish to marine environments．The dominant species are Bicornucythere sp.，Spinileberis quadriaculeata and Aurila spinifera s.l．Five ostracode biofacies are recognized：AL，AB，BA，B and BT．Biofacies AL（sandy bay coast with Zostera）is present in the upper part ofthe member，which is distributed in the western part of the study area．Biofacies AB，BA，B and BT are distributed in upward sequence in the central to eastern parts of the study area．Based on fossil ostracode biofacies and sedimentary facies，the depositional environments of the Akasawa Silt Member in its central to eastern parts are interpreted as anoxic muddy bay，middle bay influenced by tidal currents，and enclosed middle bay in upward sequence．