A method of the present invention comprises the following steps: a step (a) for preparing a device, which is provided with a cathode chamber (102), an anode chamber (105) and a solid electrolyte membrane (106), and wherein the cathode chamber is provided with a cathode electrode (101), the anode chamber is provided with an anode electrode (104), the anode electrode comprises a nitride semiconductor region (302), a part of the surface of the nitride semiconductor region is covered with a nickel or titanium region (303), the region (303) is in contact with the nitride semiconductor region, the cathode chamber holds a first electrolyte solution (107), the anode chamber holds a second electrolyte solution (108), the cathode electrode is in contact with the first electrolyte solution, the anode electrode is in contact with the second electrolyte solution, the first electrolyte solution contains carbon dioxide, the cathode electrode is electrically connected to the anode electrode, the anode electrode comprises an anode electrode terminal (111), and the region (303) is separated from the anode electrode terminal; and a step (b) for reducing the carbon dioxide in the first electrolyte solution by irradiating at least a part of the nitride semiconductor region with light having a wavelength from 250 nm to 400 nm (inclusive), while also irradiating the region (303) with the light.