NOVELTY - A methane-fermentation processing apparatus has solubilization tank (24), souring tank (29) and methane fermentation tank (10). The tank (24) mixes and solubilizes alkali with specific to-be-processed material. The tank (29) mixes solubilized slurry with specific to-be-processed material, and performs souring of mixture. The tank (10) performs methane fermentation of organic-acid containing solubilized slurry.    USE - For solubilizing organic substances such as plant waste and animal waste to produce methane gas.    ADVANTAGE - The methane-fermentation processing apparatus consumes reduced quantity of alkali or acid, and hence is inexpensive. The apparatus effectively performs fermentation of organic waste, and collects large quantity of biogas. The biogas is effectively utilized as fuel, and energy is recovered as electricity or warm water. The size of methane fermentation processing apparatus can be minimized. The discharge of methane fermentation sludge is decreased, and amount of sludge to-be-treated is also reduced.    DETAILED DESCRIPTION - A methane-fermentation processing apparatus has solubilization tank (24), souring tank (29) and methane fermentation tank (10). The solubilization tank mixes and solubilizes an alkali with to-be-processed material (22) having plant substance as main constituent. The souring tank mixes alkaline solubilized slurry produced by the solubilization tank, with to-be-processed material (27) having animal substance as main constituent, and performs souring of the mixture. The methane fermentation tank performs methane fermentation of organic-acid containing solubilized slurry produced by souring tank.    An INDEPENDENT CLAIM is included for methane-fermentation processing method.    DESCRIPTION OF DRAWING(S) - The figure shows flow chart of the methane fermentation system.    methane fermentation tank (10)    to-be-processed material having plant substance as main constituent (22)    solubilization tank (24)    alkali addition line (25)    to-be-processed material having animal substance as main constituent (27)    souring tank (29)    pH meter (42)    acid addition line (44)