NOVELTY - Digestion gas emitted by methane fermentation of organic waste is compressed, supplied to an absorption tower (3), and contacted with high-pressure water. Methane gas is purified, added with hydrogen, supplied to the a catalyst-filled column (8), and converted into water and oxygen. The concentration of methane, carbon dioxide, and/or hydrogen sulfide in purified gas is detected. When the concentration is not in the regulated range, purified gas at the absorption-tower exit is bypassed by a flow-path switching valve (40).    USE - Deoxidization of digestion gas emitted from methane-fermentation of organic waste.    ADVANTAGE - The method provides purified gas with complete removal of sulfur type impurities e.g. hydrogen sulfide, without requiring a high-temperature process. Most of carbon dioxide in digestion gas and sulfur type impurities are previously separated using a high-pressure water absorption method, and since purification of methane gas is carried out, simultaneous removal of sulfur type impurities and oxygen is not required. Sufficient deoxidization is achieved just by adding hydrogen and advancing catalytic reaction at normal temperature, without catalytic reaction requiring a high temperature of 300-450 degrees C, when removing oxygen remaining in purified gas.    DETAILED DESCRIPTION - Digestion gas emitted by methane fermentation of organic waste is compressed and pressure-increased with a compressor (2a). The pressure-increased digestion gas is supplied to an absorption tower and contacted with water in high-pressure state. Carbon dioxide and sulfur type impurities contained in the digestion gas are dissolved in high-pressure water and separated, and methane gas is purified. Hydrogen is added to purified methane gas, supplied to the a column filled with catalyst (7), and converted into water and oxygen remaining in purified gas by catalytic reaction. The concentration of methane, carbon dioxide, and/or hydrogen sulfide in purified gas at absorption-tower exit is detected. When the concentration is not in the regulated range, purified gas at the absorption-tower exit is bypassed by a flow-path switching valve except the catalyst column. When the concentration is in the regulated range, gas at the absorption-tower exit is supplied to the catalyst column by the flow-path switching valve. An INDEPENDENT CLAIM is included for digestion-gas deactivator consisting of a compressor, an absorption tower, a hydrogen supplier, a catalyst column, and a gas-concentration detector (30) and a flow-path switching valve provided between the catalyst columns.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the deactivator. (Drawing includes non-English language text)    Compressor (2a)    Absorption tower (3)    Water electrolysis apparatus (6)    Catalyst (7)    Catalyst column (8)    Gas-concentration detector (30)    Flow-path switching valve (40)