NOVELTY - Biogas purification system comprises biological desulfurization equipment (11), and oxygen analyzer (21) and gas meter (22) provided at discharge pipeline (13) of the desulfurized biogas. An air injection apparatus (20) is connected with the introduction pipeline (12) to inject untreated biogas and air. When the oxygen analyzer's measured value is above the set value, the control apparatus is allowed to stop air injection.    USE - For purification of biogas obtained by methane fermentation of organic waste generated from livestock farming excrements, sewage works or industrial-wastewater-treatment installation such as sludge and organic refuse, by action of microorganisms and removing impurities such as hydrogen sulfide contained in biogas.    ADVANTAGE - Oxygen supply to aerobic sulfur-oxidizing-bacteria group used in biological desulfurization equipment is performed appropriately, and biogas purification system having maintained activity and high desulfurization capability is provided.    DETAILED DESCRIPTION - The biogas purification system comprises biological desulfurization equipment, and oxygen analyzer and gas meter provided at discharge pipeline of the desulfurized biogas. The biogas produced from methane-fermentation tank is introduced into the biological desulfurization equipment, and desulfurized by sulfur oxidizing bacteria. The air injection apparatus is connected with the pipeline to inject untreated biogas and air. The air is injected at constant and basic injection rate using air injection apparatus, after adjustment, based on the measured value of the gas meter. When the oxygen analyzer's measured value is above the set value, the control apparatus is allowed to stop air injection.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the biogas purification system. (Drawing includes non-English language text).    desulfurization equipment (11)    introduction pipeline (12)    discharge pipeline (13)    air injection apparatus (20)    oxygen analyzer (21)    gas meter (22)