A “Miocene” molluscan fauna of the Yunokogawa Formation is reexamined taxonomically and its paleobiogeographic implications are discussed. The fauna is composed of three species of Gastropoda and 18 species of Bivalvia. The new data indicate that the Yunokogawa molluscan fauna is not of Miocene age, but is referable to the Omma-Manganji Fauna of Pliocene—Early Pleistocene age. No warm-water species are included in the Yunokogawa molluscan fauna. This is probably because the fauna is older than ca. 3.5 Ma or because the influence of warm-water (the Paleo-Tsushima Current) in the Paleo-Tsugaru Strait was much weaker than today. The following taxa are described taxonomically: Fusitriton aff. oregonensis (Redfield), Anadara (Anadara) amicula amicula (Yokoyama), Chlamys (Leochlamys) foeda (Yokoyama), Thracia (Homoeodesma) kakumana (Yokoyama), Protocallithaca Nomura, 1937, and Protothaca (Protocallithaca) adamsii (Reeve).