NOVELTY - Biogas is generated by anaerobic fermentation of organic waste in fermenter. Biogas pressure is increased by gas-compression machine (402a,402b), and biogas is contacted with high-pressure water. Biogas impurities are dissolved in water and separated, to obtain purified gas. The gas pressure of fermenter is measured, and rotation number of compression machine is increased and decreased to maintain constant gas pressure.    USE - For purifying biogas generated by anaerobic fermentation of organic waste, such as sewage sludge generated in sewage works, foodstuffs waste materials, livestock excrements, and organic wastewater.    ADVANTAGE - The method efficiently purifies biogas, removes carbon dioxide and sulfur impurities, and provides purified gas of highly concentrated methane, using a compact and economical installation. Biogas is removed stably from the anaerobic fermenter, and fluctuation of biogas generation amount is reduced.    DETAILED DESCRIPTION - Biogas is generated by anaerobic fermentation of organic waste in an anaerobic fermenter. The pressure of biogas from the fermenter is increased by a gas-compression machine (402a,402b), and biogas is contacted with water in high-pressure state in an absorption tower (404). Carbon dioxide and sulfur group impurities contained in biogas are dissolved in the high-pressure water and separated from biogas, to obtain purified gas containing highly concentrated methane. The gas pressure of fermenter is measured, and the rotation number of gas-compression machine is increased and decreased such that the gas pressure is maintained constant based on the measured value.    An INDEPENDENT CLAIM is included for biogas refinery processing installation equipped with an anaerobic fermenter, a gas-compression machine, a biogas refiner which obtains purified gas, and a controller (403) for increasing and decreasing rotation number of gas-compression machine.    DESCRIPTION OF DRAWING(S) - The figure shows the structure of biogas refinery processing installation. (Drawing includes non-English language text).    gas-compression machine (402a,402b)    controller (403)    absorption tower (404)    de-humidifier (405)    water supply tank (406)