Gamma-ray radiation induced change in the electrical conductivity of the proton-conducting polymer, perfluorosulfonic acid membrane, was investigated. The electrical conductivity increased with increasing the absorption dose in linear form at 300 K and hereafter gradually increased up to be higher by about three orders of magnitude. The conductivity greatly depended on the humidity in vacuum or air. Moreover it was observed by optical absorption measurement that radiation induced defects such as fluorocarbon and peroxy radicals and C=O groups increased with increasing the dose. Therefore, the increase of the conductivity is attributed to conduction due to protons produced by absorption of H2O with the radiation induced defects.