Seventy two species groups of Ostracoda were identified from 33 samples obtained from the drilling core 'A', excavated in the southern Osaka Plain, southwest Japan. Q-mode cluster analysis of ostracode faunas in this core reveals six biofacies (PS, SBm, LS, PL, CL and LC). This study provided ostracode evidence revealing temporal variation in depositional environment and relative sea level throughout the Holocene. In the southern Osaka Plain, the sea level rose rapidly from the period between ca. 9, 000 and ca. 6, 000 cal yr BP. The maximum sea-level highstand was at ca. 6, 000-5, 600 cal yr BP, after which the sea level has gradually fallen to its present one. This trend in sea-level change is well correlated to those of central Osaka Plain and Osaka Bay, suggesting that the relative sea-level changes were widespread throughout the Osaka sedimentary basin. Changes of salinity, distance from shoreline and influence by coastal current and waves are well correlated with the observed relative sea-level changes.