Six ostracode species were discriminated from the latest Eocene-earliest Oligocene marine sediments in Kurashiki City, Okayama Prefecture along the Seto Inland Sea in southwestern Japan. The ostracode assemblage is characterized by Munseyella setouchiensis and Acanthocythereis kurashikiensis sp. nov. They inhabited the lower sublittoral zone, affected by ocean waters. The new record from the Kurashiki area indicates that the eastern area of the Seto Inland Sea was under a cooler marine climate than the Kyushu region from the late Middle Eocene to earliest Oligocene. All the species including a new species, Acanthocythereis kurashikiensis sp. nov., are described systematically.