Fossil ostracode assemblages of alluvium sediments are investigated in the western part of Izumo Plain to discuss detailed paleoenvironments of the paleo Shinji Bay. Four samples from core in Lake Jinzai and seven samples from core in Hishine-Ike were selected for this study. Thirty-four genera and 71 species fossil ostracodes are recognized. Fossil ostracode assemblages in this study are close to those of the recent Lake Nakaumi. Two ostracode species, Bicornucythere bisanensis and Spinileberis quadriaculeata, are mostly dominated in both core. It indicates that Lake Jinzai and Hishine-Ike were located in central part of the paleo Shinji Bay in 2-9 water depth between 9500-8000 yr. BP. The bay mouth ostracode fauna in mud bottom in the recent Nakaumi occurred in the middle part of the core from Hishine-ike. It shows that the reduction of the paleo Shinji Bay occurred at lease one time. Spinileberis quadriaculeata shows very low percentage in core from Hishine-Ike, though it represents relatively high in core from Lake Jinzai. This suggests that southwestern part of the paleo Shinji Bay was more oxic conditions compared with northwestern part.