Fission-track age determinations using both internal and external surfaces of zircons for 23 core samples of the "Green Tuff" were carried out. These samples were collected from 21 wells drilled for the petroleum exploration in Niigata Prefecture.After the detail examinations for grain ages of each sample using histograms and age spectra, fission-track ages are interpretated. Fission-track ages of the "Green Tuff" are divided into two age groups (15.5 to 17.1 Ma and 13.5 to 13.8 Ma). The older group is recognized in all three districts (Kita-kambara, Mitsuke and Yoshii districts) and corresponds to the Tsugawa stage or the lowest part of the Nanatani stage. The younger group, corresponding to the uppermost part of the Nanatani stage, is relatively restricted.Grain ages show the presence of zircons derived from basements in the Kita-kambara and Yoshii districts. No basement zircons and younger fission-track ages of some wells such as Sanjyo SK-1 indicate that volcanism was active for a long period in the Mitsuke district.An analysis of grain ages in the Mitsuke oil field using Brandon (2002) show felsic volcanism was active around 23 Ma, same as the Sado Island and Asahi Mountains.Reset of fission-track ages of both surfaces and distribution of grain ages give us some information on past thermal conditions. For example, the thermal alteration in the Yoshii district occurred around Yoshii SK-10D and started just after the eruption of rhyolite can be inferred.