Electrochemical reduction of CO2 under high pressure (<50 atm) on a Pt gas diffusion electrode (Pt-GDE) was studied. When the Pt catalyst layer was directed toward the CO2 gas phase (the reverse arrangement), methane was produced as the main CO2 reduction product at high faradaic efficiency and at large partial current density. Ethanol, ethylene, ethane, carbon monoxide, and formic acid were also produced. The effects of the Pt catalyst, electrode arrangement, CO2 pressure, reaction temperature, and hydrogen pretreatment of the electrode were investigated for this electrocatalytic reaction. Faradaic efficiencies of the formation of CO2 reduction products such as methane, carbon monoxide, and formic acid depended significantly on the CO2 pressure and temperature. The yield of CO2 reduction products was increased by hydrogen pretreatment of the Pt-gas diffusion electrode.