NOVELTY - The system has primary solubilization tank (PST) to solubilize an organic waste (1). A secondary solubilization tank solubilizes primary solubilized residuum obtained by solid-liquid separation of primary solubilized material from PST. A dehydrator separates primary and secondary solubilized materials into solids and liquids. A solubilized liquid mixture is methane-fermented in methane-fermentation tank.    USE - For methane fermentation of waste materials such as garbage.    ADVANTAGE - The output of solubilized residuum and methane fermentation sludge can be reduced, thereby reducing final treatments necessary for incineration of sludge. The amount of methane formed can be increased by controlling decomposition of organic acid in the solubilization tanks. The size of secondary solubilization tank can be reduced.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for methane-fermentation of organic waste. Organic waste is solubilized in a primary solubilization tank (3), and primary solubilized material (30) is formed. The primary solubilized material is subjected to solid-liquid separation, and primary solubilized residuum (7) and primary solubilized liquid (6) are formed. The primary solubilized residuum is solubilized in a secondary solubilization tank (22), and secondary solubilized material is formed. The secondary solubilized material (31) is subjected to solid-liquid separation, and secondary solubilized residuum (25) and secondary solubilized liquid are formed. The primary and secondary solubilized liquids are then mixed and introduced into a methane-fermentation tank where methane-fermentation is performed.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of methane-fermentation processing system.    Organic waste (1)    Primary solubilization tank (3)    Dehydrator (5)    Primary solubilized liquid (6)    Primary solubilized residuum (7)    Buffer tank (8)    Methane-fermentation tank (10)    Sludge (15)    Secondary solubilization tank (22)    Secondary solubilized residuum (25)    Primary solubilized material (30)    Secondary solubilized material (31)