Planktonic foraminiferal age of the Miocene sequence in the Iwadono Hills of central Japan is determined in reference to recent advances in Miocene planktonic foraminiferal biostratigraphy of Japan. The interval from the Godo through the Negishi to the lower part of the Shogunzawa Members of the Iwadono Formation yields 55 species of planktonic foraminiferal fossils. The Godo Member is assigned to planktonic foraminiferal zone N.8. The Negishi Member and the lower part of the Shogunzawa Member are confined to zones N.10 to N.13. Nine foraminiferal biohorizons were detected, seven of which enable us to determine a detailed age-thickness model for the sequence. The age-thickness model indicates a remarkably slow sedimentation rate in the Negishi Member (less than 9 mm/kyr). Furthermore, a relationship is established between the foraminiferal biostratigraphy and other stratigraphic data (diatoms, calcareous nannofossils, and radiometric ages) in the area. This relationship agrees well with the age-correlation chart of Japanese Neogene sediments.