Fission-track (FT) dating was carried out on zircon crystals separated from various volcanic rocks of the Miocene Nijo Group, which is part of the Setouchi volcanic rocks. Twelve FT ages from a wide stratigraphic range were indistinguishable at the 2σ level and made a cluster around 15Ma. Accuracy of the FT ages was confirmed by semi- and confined-FT length measurements, as well as by repeated dating of the age standard (Fish Canyon Tuff). Our FT ages demonstrate that the Nijo Group was formed within a very short period around 15 Ma. It is concluded, based on the present results and reliable radio-metric ages from other areas, that the Setouchi volcanic activity occurred around 15 Ma. The well-known view that the intermediate to mafic rocks of the Setouchi volcanic rocks were formed around 12Ma while the felsic rocks around 14Ma is highly questionable. Additionally, it is confirmed that the FT method is useful in dating intermediate, glassy volcanic rocks if they contain zircon crystals regardless of origin. Furthermore, correlation of sporadically exposed Middle Miocene felsic pyroclastic flow deposits in the northern Kii Peninsula region is suggested from the similarity of their lithologies, FT ages, and the proportion of reddish zircon grains to colorless ones.