Pushed by human activities, the atmospheric CO2 concentration has reached levels which have made extreme weather events more likely, and which threaten permanent climate changes for the world. In order to combat this, CO2 emissions must be reduced significantly. Electricity production and industry are two sectors which contribute greatly to the production of CO2, thus a reduction in the amount of CO2 produced by thermal power plants and factories could make a significant contribution to combatting climate change. The electrochemical reduction of CO2 in those sources is considered very likely to be a solution to the problem. In this study, the activity of a Pt oxide electrode in the electrochemical reduction of CO2 was investigated in a sulfuric acid solution. Pt oxide electrodes have shown superior activity for the methanol oxidation reaction, which is the reverse reaction of CO2 electrochemical reduction. Cyclic voltammetry of the Pt oxide electrode in a CO2-saturated H2SO4 solution showed a definite anodic peak at 0.6-0.8 V vs. SHE, which was not observed in an Ar-deaerated electrolyte. Thus, it was determined that the anodic peak could be related to the re-oxidation of the reduction product of CO2 during cathodic polarization. The activity of the Pt oxide electrode for CO2 reduction was much higher than that of the Pt electrode. It was concluded that the residual oxygen, which was hardly detected in the Pt electrode, improved the activity for CO2 electrochemical reduction on the Pt oxide electrode. Gas chromatography-mass spectrometry of the electrolytic solution after CO2 reduction revealed that the reduction product was mainly CH3OH. These results should be very useful for developing a new electrochemical reduction system for converting CO2 into CH3OH.