We present a new fission track age for a volcanic rock that formed during the opening period of the Japan Sea. A basal rhyolite (partly perlitic) flow of the Fuyube Formation on the Kodomari Peninsula yielded a zircon fission track age of 15.3 ± 0.9 Ma, determined by the external detector method. This result is compatible with published age data for underlying and overlying formations, and can be interpreted to represent the emplacement age of the lava flow. A chronostratigraphic model is given for the Lower to Middle Miocene sequence in the northern part of the Tsugaru district, in which the age of the Fuyube Formation is considered to be about 16 Ma.