Based on our previous finding that there is a correlation between proton conductivity of ionomers and carbon oxidation degradation in the cathode, we have focused on further investigating the correlation between proton conductivity of ionomers and carbon oxidation by varying ionomers. Carbon oxidation was evaluated in . terms of increase in pore volume using a F1B-SEM technique followed by reconstructing SEM images to three-dimensional images. We considered degradation under both the high RH • condition, where the difference in proton conductivity of ionomers • was less effective, and the low RH condition, where difference in proton conductivity of ionomers was more effective, by using different kinds of ionomers with different proton conductivity. Consequently, dependence of proton conductivity on carbon oxidation degradation in the low humidity condition was observed, and controlling the proton conductivity of ionomers was found to be important especially under low humidity.