Single zircon U-Pb analytical technique by SIMS (secondary ion mass spectrometry) was applied to metasediment from Oki-Dogo Island, southwest Japan. ^<207>Pb/ ^<206>Pb age of zircons comprise three generations; 2.3, 2.0, and 1.7 Ga. Well-rounded crystal shape of the analyzed zircons suggests that the sediments are detritus of a continental land mass which was Proterozoic in age. The age dispersion may be attributed to at least three individual magmatic activities in the Proterozoic continent.