TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Process: The processed liquid obtained by solid-liquid separation is subjected to activated sludge treatment. A secondary solid-liquid separation process is carried out to obtain secondary treated water and secondary sludge. At least one potion of the solid substance and the secondary sludge are biologically solubilized with thermophilic bacteria. At least one portion of the secondary sludge is subjected to a secondary solubilization process. The solid-liquid separation processes and the activated sludge processing are implemented according to any one process of membrane-separation activated sludge treatment. The organic solid waste is crushed before methane-fermentation. The solubilization process involves aerobic heat-processing at 60-70degreesC. The organic solid waste is pre-treated. The moisture content of the organic solid waste is adjusted prior to methane-fermentation. EXAMPLE - Bean curd lees (in weight parts) (1), Japanese noodles (1) and bread (1) were mixed and processed for 3 minutes. The mixture was taken in a raw water tank after crushing to about 2 mm of mean particle diameter. Tap water was added and the total solid concentration was adjusted to about 10% (w/v). At the rate of 1 L/day, raw water was supplied to a methane-fermentation treatment tank. The hydraulic residence time was set as 20 days at 37degreesC, and methane-fermentation was carried out. The fermented material was solid-liquid separated in a settling tank. The obtained solid substance was introduced into a solubilizing tank at 1.6 L/day. The solubilization process was carried out at 70degreesC with a residence time of 1 day. A solubilized liquid was returned to the methane-fermentation treatment tank. The sum of pouring volume to the methane-fermentation treatment tank and the settling tank was 2.6 L/day. The supernatant from the settling tank was obtained at 1 L/day. The process was performed for 2 months. The suspended solid weight was 8 g/day. The amount of methane gas obtained by the methane-fermentation treatment tank was 52 L/day.