Electrons are introduced at the cathode (1) side of a cell (10), promoting reduction of carbon dioxide. Meanwhile, at the anode (2) side, oxidation of methane is promoted by oxide ions transported through a porous GDC membrane (3). Once carbon monoxide and oxide ions are generated, these, together with methane gas, are transported to the anode (2), where carbon monoxide and hydrogen are generated. Also, at the anode (2), Cu and oxide ions react, promoting oxidation of the methane gas.