NOVELTY - A liquid organic mixture prepared from organic waste is subjected to heating/pressurization. A gas-liquid mixed phase containing gas comprising methane and/or hydrogen gas, is produced. The obtained gas-liquid mixed phase is decarboxylated, and gas containing methane and/or hydrogen gas is recovered. The separated liquid phase is oxidation-decomposed, and obtained gas-liquid mixed phase is vapor-liquid separated.    USE - For manufacturing fuel gas from organic wastes, such as garbage, paper, plastic, wood chips, bamboo and grass wastes, straw fiber, vegetable waste, rubber, food-processing waste material, livestock farming waste material, forest coppiced timber, agriculture-and-forestry waste material, marine product waste material, biological waste material, and other factory waste materials.    ADVANTAGE - The manufacturing method enables recovery of fuel gas, electric power, and heat energy from solid and liquid organic wastes. The method serves as a new technique for recycling. The restrictions in processing system are reduced. Solid and liquid organic wastes are economically converted to useful methane series fuel gas with high energy conversion efficiency. The usage amount of fossil fuel is reduced, and environmental pollution is prevented as generation of harmful substances, such as dioxins, is prevented.    DETAILED DESCRIPTION - A liquid organic mixture prepared from organic waste is subjected to heating/pressurization processing in reactor (I) at a pressure which maintains a liquid phase at 100degreesC or more or a supercritical phase. A gas-liquid mixed phase containing a decomposable gas comprising methane and/or hydrogen gas, is produced in reactor (II) using the processed substance in the presence of a catalyst which supports a metal and/or a metallic compound as an active ingredient, and/or inorganic packed material at 100degreesC or more. The obtained gas-liquid mixed phase is decarboxylated and released. A gas containing methane gas and/or hydrogen gas, is recovered. The separated liquid phase containing residual organic component and ammonia component is oxidation-decomposed in the presence of a gas containing oxygen in an amount more then the theoretical amount, at 100degreesC or more, in reactor (III). The obtained gas-liquid mixed phase is then subjected to vapor-liquid separation.    DESCRIPTION OF DRAWING(S) - The figure shows the flow sheet of the manufacturing method of fuel gas. (Drawing includes non-English language text).