Solid polymer electrolyte membrane has been used as electrolyte in polymer electrolyte fuel cells (PEFC) has high ionic conductivity. The hydrogen peroxide made by the reaction in a fuel cell may degrade the conductivity in PEFC. It is necessary to investigate the degradation behavior of the membrane in the environment where the hydrogen peroxide exists. The membrane that was immersed in hydrogen peroxide was used as a sample of each measurement. The measurement of the proton diffusion rate in the membrane was performed for the measurement of a proton conductivity. The current-voltage curve measurement was performed for the investigation of a fuel cell performance. The measurement of a proton diffusion rate applied to the degradation process in the membrane was effective to evaluate the conductivity in the fuel cell.