NOVELTY - Biogas is refined by introducing methane-containing biogas and carbon dioxide from methane into biogas refining apparatus main body provided with refining part which has electroconductive microorganism carrier impregnated at methane-bacterium-containing liquid, applying electric potential to incline carbon dioxide-methane balanced group to methane formation side of electroconductive microorganism carrier, making gas-liquid contact of methane-bacterium-containing liquid and biogas in the refining part, and refining to stabilize methane concentration in biogas beyond a constant value.    USE - A biogas refining method for fuel.    ADVANTAGE - Carbon dioxide emission is reduced, and methane concentration in biogas is stabilized beyond a constant value.    DETAILED DESCRIPTION - Biogas is refined by introducing methane-containing biogas and carbon dioxide from methane into biogas refining apparatus main body (1) provided with refining part (2, 22) which has electroconductive microorganism carrier impregnated at methane-bacterium-containing liquid, applying electric potential to incline carbon dioxide-methane balanced group to methane formation side of electroconductive microorganism carrier, making gas-liquid contact of methane-bacterium-containing liquid and biogas in the refining part, and refining to stabilize methane concentration in biogas beyond a constant value using methane formation rate rising according to the fall of methane concentration in biogas. INDEPENDENT CLAIMS are included for:    (1) biogas refining apparatus comprising biogas refining apparatus main body (1), biogas inlet (10, 25), refining part (2, 22), and electric potential application pole (3, 23); and    (2) biogas system comprising methane fermentation tank consists of desulfurization column, biogas refining apparatus main body, biogas inlet, biogas discharge port (11, 26), and electric potential application pole.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a biogas refining apparatus. (Drawing includes non-English language text).    Biogas refining apparatus main body (1)    Refining part (2)    Electric potential application pole (3)    Biogas inlet (10)    Biogas discharge port (11)