Geologic age of the "Miocene" Iwaya Formation in Awajishima Island is examined by means of calcareous nannofossils and dinoflagellate cysts. Calcareous nannofossil flora indicates biozone NP 17 to NP 20 or CP 14 b to CP 15 b and a late Middle Eocene to Late Eocene age. The dinoflagellate cyst assemblages also show approximately the same age. These results reveal that the Iwaya Formation is the constituent of the Eocene-Oligocene Kobe Group.Molluscan assemblages resembling those from the Iwaya Formation have been known from the "Setouchi Miocene Series" in the coastal area of the eastern Seto Inland Sea. Molluscan fauna and stratigraphic correlation of the "Miocene" in this area should also be re-examined on the basis of precise geochronologic data.