NOVELTY - Methane gas is purified by compressing sewage gas, increasing pressure with a compressor, supplying sewage gas to an absorption tower (3), contacting sewage gas and high-pressure water in absorption tower, dissolving carbon dioxide and sulfur-type impurity contained in sewage gas, in high-pressure water, and separating carbon dioxide and sulfur-type impurity from the sewage gas. Hydrogen is added to purified methane gas. Oxygen in purified gas which is supplied to catalyst tower (8) is converted to water.    USE - Deoxidization of sewage gas emitted by methane fermentation of organic waste.    ADVANTAGE - The method enables deoxidization of sewage gas, without catalytic reaction at high temperature. The replacement time of catalyst in catalyst tower during deoxidization of sewage gas is efficiently judged.    DETAILED DESCRIPTION - Methane gas is purified by compressing sewage gas, increasing pressure with a compressor, supplying sewage gas to an absorption tower, contacting sewage gas and high-pressure water in absorption tower, dissolving carbon dioxide and sulfur-type impurity contained in sewage gas, in high-pressure water, and separating carbon dioxide and sulfur-type impurity from the sewage gas. Hydrogen is added to purified methane gas. Oxygen in purified gas which is supplied to catalyst tower is converted to water. The temperature of purified gas in catalyst tower is measured with respect to temperature of purified gas discharged from the catalyst tower. An INDEPENDENT CLAIM is included for deactivator, which has compressor which compresses sewage gas and the pressure of sewage gas is increased, absorption tower to which sewage gas and water are supplied, a hydrogen supply unit which adds hydrogen to purified methane gas, catalyst tower filled with catalyst which converts oxygen in purified gas to water, and multiple temperature measuring unit.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the deactivator. (Drawing includes non-English language text)    Absorption tower (3)    Water supply tank (4)    Water electrolysis apparatus (6)    Catalyst tower (8)    Dehumidifier (9)