In the eastern margin of the Ashio Mountains, Tochigi Prefecture, central Japan, middle Miocene volcanic rocks are distributed with few reliable chronological data. This paper includes newly measured whole rock K-Ar dates of the Kazamiyamada Andesite in the Utsunomiya area and the Hinata Formation in the Kanuma area. The pyroxene andesite lavas of the Kazamiyamada Andesite give 15.78±0.64, 16.61±0.84, 15.26±0.40 and 15.02±0.39 Ma, which are thought to be the eruption age. The basaltic andesite lavas of the Hinata Formation yield 16.8±2.1, 17.3±2.0, 15.6±2.2 and 15.3±2.4 Ma, and these lavas correspond to the Kazamiyamada Andesite by the lithofacies and stratigraphic evidences. This means chronologically the Kazamiyamada Andesite and the Hinata Formation are correlated to the hiatus between the Yamanouchi and Motegi Formations in the Motegi area in western part of the Yamizo Mountains. According to lithofacies and microfossil data of overlying marine beds, Motegi Formation, Nagaoka/Oya Formations in the Utsunomiya area and Fukaiwa Formation in the Kanuma area are correlated to each other.