NOVELTY - A carbon dioxide-reducing device (300) comprises a cathode tank for storing an electrolyte (307) containing carbon dioxide, an anode tank for storing an electrolyte (308), a proton-permeable membrane positioned in the section connecting the two tanks, a cathode electrode (301) inside the cathode tank, and an anode electrode (304) inside the anode tank. The cathode electrode has a carbon dioxide reduction reaction region containing metal or its compound. The anode electrode has a photochemical reaction region containing nitride semiconductor.    USE - Device is used for reducing carbon dioxide for preparing organic compound e.g. methanol, ethanol, acetaldehyde, formic acid, methane, and ethylene, and carbon monoxide (all claimed), and artificial photosynthesis.    ADVANTAGE - The device efficiently and economically reduces carbon dioxide without using an external power supply.    DETAILED DESCRIPTION - A carbon dioxide-reducing device comprises a cathode tank for storing an electrolyte (307) containing carbon dioxide, an anode tank for storing an electrolyte, a proton-permeable membrane positioned in the section connecting the two tanks, a cathode electrode inside the cathode tank, and an anode electrode inside the anode tank. The cathode electrode has a carbon dioxide reduction reaction region containing metal or its compound. The anode electrode has a photochemical reaction region containing nitride semiconductor. The region of the anode electrode has a layered structure comprising gallium nitride layer and magnesium-doped aluminum-gallium nitride (AlxGa1-xN, where x is more than 0-0.25) layer with magnesium content of 1x 1015-1x 1019/cm3 of layer. The anode electrode is positioned such that irradiation of the aluminum-gallium nitride layer with light is possible. Both electrodes are electrically connected without using an external power source. An INDEPENDENT CLAIM is included for method of reducing carbon dioxide by carbon-dioxide reducing device.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic structure of carbon dioxide reducing device. (Drawing includes non-English language text)    Carbon dioxide-reducing device (300)    Cathode electrode (301)    Cathode cell (302)    Anode electrode (304)    Anode cell (305)    Electrolyte solutions (307,308)