To provide a method for using lignocellulose-based biomass, which enables the yield of a combustible gas consisting mainly of methane and containing neither carbon monoxide nor tar from the lignocellulose-based biomass such as lumbers or the like not through a high-temperature condition requiring an expensive heat-resistant material and accompanies no 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 yielded 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 generate the combustible gas consisting mainly of methane.

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