U-Pb age of zircon separated from the Ashio rhyolitic welded tuff　was determined, and whole-rock K-Ar ages with different alteration degrees were also dated. The U-Pb age of zircon in the least altered tuff was determined to have 10.25±0.16 Ma, and its whole-rock K-Ar age was dated at 11.3±0.4 Ma. This Miocene age seems to be the youngest among the major rhyolitic activities in the Oya-Ashio mine region; the youngest one brought up the largest amount of copper production in Japan toward the Ashio mine.