NOVELTY - Reducing carbon dioxide involves a) making carbon dioxide reducing cell including: proton exchange membrane inserted between cathode and anode chamber; cathode set inside cathode chamber to contact electrolytic solution; and anode set inside anode chamber to contact second electrolytic solution, anode (10A) having first semiconductor layer (11) constituted by a nitride semiconductor comprising aluminum gallium nitride layer, and n-type gallium nitride layer, and second semiconductor layer (12); and (b) irradiating anode with light to reduce carbon dioxide in first solution on cathode.    USE - For reducing carbon dioxide to yield at least one of carbon monoxide, formic acid, methane, ethylene, methanol, ethanol, isopropanol, and allyl alcohol; for converting the carbon dioxide to an organic substance (claimed).    ADVANTAGE - The method allows reduction of carbon dioxide on the cathode to produce a reduction product with high efficiency. Using the method, the mechanism for tracking sun is used to allow a carbon dioxide reducing cell to face to sun, and a light-concentrating device is used to irradiate the anode (photochemical electrode) of this cell with light, the energy density of which is heightened per unit area, thus increasing the effects of the cell. Through the mechanism of the driving section, the carbon dioxide reducing cell set onto the stand section is moved in a rotational direction (corresponding to the azimuth) and an angular direction (corresponding to the elevation angle) so that the front of the cell can be varied to be along an arbitrary direction and have an arbitrary angle. The control section controls the movement of the driving section to match the front of the carbon dioxide reducing cell with the position (the azimuth and the elevation angle) of the sun at the time of the control, so that the carbon dioxide reducing cell faces to the sun. By using a photochemical electrode (anode), constitution in which a first semiconductor layer made of a nitride semiconductor material and a second semiconductor layer having a pn junction structure are stacked from the light-irradiated surface side of the electrode, recombination of photoexcited carriers is restrained, and further the photo voltaic power generated in the anode (photochemical electrode) is heightened, so as to improve the reduction efficiency of carbon dioxide on the cathode.    DETAILED DESCRIPTION - A carbon dioxide reducing method involves a) preparing a carbon dioxide reducing cell including: a1) a cathode chamber that holds a first electrolytic solution containing carbon dioxide; a2) an anode chamber that holds a second electrolytic solution; a3) a proton exchange membrane inserted between the cathode chamber and the anode chamber; a4) a cathode set inside the cathode chamber to contact the first electrolytic solution and, the cathode having at least one component selected from copper, gold, silver and indium, respective alloys of these elements, and respective metal compounds of these elements on a surface of the cathode; and a5) an anode set inside the anode chamber to contact the second electrolytic solution, the anode (10A) having a first semiconductor layer (11) constituted by a nitride semiconductor comprising an aluminum gallium nitride (AlxGa1-xN) layer (13)(where x is 0-0.25), and n-type gallium nitride (GaN) layer (14), and a second semiconductor layer (12) constituted by a semiconductor having a pn junction structure and electrically connected to the first semiconductor layer through its p-type semiconductor layer, and further the anode being electrically connected to the cathode through no external power source; and (b) irradiating the anode with light to reduce carbon dioxide contained in the first electrolytic solution on the cathode. INDEPENDENT CLAIMS are included for the following:    (1) a carbon dioxide reducing cell for reducing carbon dioxide using light, comprising: the cathode chamber (a1), the anode chamber (a2), proton exchange membrane (a3), cathode set (a4) and the anode set (a5); and    (2) a carbon dioxide reducing apparatus comprising the carbon dioxide reducing cell, and a sun tracking device that synchronizes a change in the facing direction of the carbon dioxide reducing cell with the movement of the sun to allow a front surface region of the anode to face to the sun, where the sun tracking device includes: a stand section that fixes the carbon dioxide reducing cell, a driving section that changes the facing direction of the carbon dioxide reducing cell, and a control section that controls the facing direction of the carbon dioxide reducing cell in accordance with the movement of the sun, where the sun tracking device is a device in which through the control section, the driving section moves the stand section to allow the front surface region of the anode of the carbon dioxide reducing cell to face to sun.    DESCRIPTION OF DRAWING(S) - The figure shows a sectional view of the anode.    Anode (10A)    First semiconductor layer (11)    Second semiconductor layer (12)    AlxGa1-xN layer (13)    GaN layer (14)    Conductive substrate (15)    Electrode part (16)    Terminal electrode part (17)