Proton irradiation damage for CuInSe2 (CIS) solar cell material was investigated using piezoelectric photothermal spectroscopy (PPTS) from the viewpoint of nonradiative transition. Three peaks at 1.01, 0.94 and 0.86 eV were observed at room temperature. The peaks at 1.01 and 0.86 eV were attributed to free exciton and the proton irradiation-induced defects, respectively. This is because the 0.86 eV peak appeared after irradiation with the proton energy of 0.38 MeV and a fluence of 1×1014 cm−2. Since the irradiation defect was clearly observed at room temperature, we concluded that the PPTS technique was a powerful tool to study the defect level in the irradiated semiconductor thin films.