To provide a method for using lignocellulose-based biomass, with which combustible gas comprising methane as a main component and containing neither carbon monoxide nor tar is obtained from the lignocellulose-based biomass not through a high-temperature condition requiring an expensive heat- resistant material and which is not followed by energy loss caused by evaporation of water contained in the lignocellulose-based biomass.

A cellulosic fiber is macerated by soaking the lignocellulose- based biomass in a cooking chemical comprising sodium hydroxide as a main active ingredient to give a mixed solution of cooked materials. The mixed solution of the cooked materials is separated into a cellulosic fiber suspension and cooking waste liquor. The cellulosic fiber suspension or a monosaccharide solution obtained by hydrolyzing the cellulosic fiber or a solution of an organic acid obtained by subjecting the monosaccharide to acid formation decomposition is treated with an anaerobic bacterium to give the combustible gas comprising methane as a main component.

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