The composite film of Nafion and the conducting polymer was prepared by dropping Nafion solution containing dispersed powder of the conducting polymer on a substrate, followed by drying. The conducting polymers used in this study were polypyrrole and polyaniline. Irradiation of the film immersed in acidic solution containing chloroplatinate and ascorbate with a 500W xenon lamp (λ > 360nm) induced Pt deposition on the composite film surface. Fluorescent X-ray analysis has revealed that amount of deposited Pt increased with irradiation time and that deposition rate on the Nafion-polyaniline composite was much higher than that on the Nafion-polypyrrole composite. Electrochemical measurements of the composite film-coated glassy carbon electrodes with and without deposited Pt have been conducted to examine their activities toward O2 reduction. Voltammetry measurements showed definite cathodic currents due to O2 reduction on deposited Pt particles. The Pt-deposited polyaniline composite possessed much higher catalytic activities than Pt-deposited polypyrrole composite because of difference in Pt amount. In order to elucidate some parameters that influence the catalytic activities, O2 reduction experiments were made by using the Nafion-polyaniline composite films prepared under different conditions including total amount of the composite film and amount ratio of polyaniline to Nafion.