Organic-walled dinoflagellate cyst assemblages which consist of 30 species are reported from the Pleistocene Sawane Formation at Shichiba, Sado Island, Niigata Prefecture, central Japan. Considering the established age of the formation as well as the dinoflagellate cyst datum levels by previous studies, 9 species are judged to be reworked from older sediments, which means that the present assemblages are heavily altered by sediment reworking during their deposition. Such sort of alteration by reworking, frequently observed in some other Late Pliocene-Pleistocene microfossil records at several localities around the Japanese Islands, may be related to intensive land uplift due to the tectonic inversion since the Late Pliocene as well as to eustatic sea level fluctuations that intensified since the Middle Pleistocene.