NOVELTY - Biogas generation system comprises: a fermenter (11) for generating a biogas containing carbon dioxide and methane by decomposing an organic waste by the action of methane bacteria; a biogas refinery (15) for condensing the methane contained in the biogas by dissolving the carbon dioxide contained in the generated biogas in a liquid; and a photoelectrochemical device (16) for generating at least one of carbon dioxide reduction products comprising methane, carbon monoxide, and formic acid from the carbon dioxide dissolved in the liquid.    USE - The system is useful for generating biogas and a carbon dioxide reduction product comprising methane, carbon monoxide, and formic acid (all claimed).    ADVANTAGE - The system: effectively uses carbon dioxide for generating biogas; utilizes light energy; and provides biogas with high purity.    DETAILED DESCRIPTION - Biogas generation system comprises: a fermenter (11) for generating a biogas containing carbon dioxide and methane by decomposing an organic waste by the action of methane bacteria; a biogas refinery (15) for condensing the methane contained in the biogas by dissolving the carbon dioxide contained in the generated biogas in a liquid; and a photoelectrochemical device (16) for generating at least one of carbon dioxide reduction products comprising methane, carbon monoxide, and formic acid from the carbon dioxide dissolved in the liquid. The photoelectrochemical device comprises: a cathode chamber for storing a first electrolyte solution containing the carbon dioxide dissolved in the liquid; an anode chamber for storing a second electrolyte solution; a solid electrolyte membrane interposed between the cathode chamber and the anode chamber; a cathode electrode provided in the cathode chamber so as to be in contact with the first electrolyte solution; an anode electrode provided in the anode chamber so as to be in contact with the second electrolyte solution; and an external power supply for applying a negative voltage and a positive voltage to the cathode electrode and the anode electrode, respectively. An INDEPENDENT CLAIM is also included for generating a biogas and a carbon dioxide reduction product, comprising: preparing the biogas generation system; placing an organic waste and the methane bacteria into the fermenter; decomposing the organic waste by the action of the methane bacteria to generate the biogas containing the carbon dioxide and the methane; dissolving the carbon dioxide contained in the generated biogas in a liquid; and reducing the carbon dioxide on the cathode electrode by applying a voltage from the external power supply through the anode electrode and the cathode electrode to the liquid in which the carbon dioxide has been dissolved, so as to give at least one of the carbon dioxide reduction products comprising methane, carbon monoxide, and formic acid on a surface of the cathode electrode.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the biogas generation system.    Fermenter (11)    Gas bag (12)    Desulfurization device (13)    Biogas refinery (15)    Photoelectrochemical device (16)