Illite K-Ar dating was conducted on phyllites from the Futase and Kawamata units of the Otaki Group in the Mitsumine area, Kanto Mountains. The K-Ar ages from the eastern part of the Futase Unit are 60-51 Ma, which are younger than previously reported K-Ar ages of 76-65 Ma. The illite fractions yielded K-Ar ages of 48-44 Ma in the western part of the Futase Unit and 46-42 Ma in the Kawamata Unit. Compared with the previous study, the measurement fractions contain relatively high K contents, and there is no significant difference in the ages obtained for each part. These results lead to the suggestion that the previous K-Ar ages are apparently older due to the influence of detrital mica and quartz grains. Integration of our results with previously published data for metamorphism of the Otaki Group indicates that the new K-Ar ages reflect the timing of the metamorphic peaks for each part of the units.