NOVELTY - Organic waste slurry is fermented in methane fermentation tank (11) by anaerobic microbe. The obtained digestive liquor is removed and processed in activated sludge tank (12). Organic acid concentration is measured in tank (11). When measured value is above predetermined value, portion of slurry having organic acid (SO) is removed from tank (11), SO is added in tank (12) and acid concentration in tank (11) is maintained.    USE - For methane-fermentation processing organic waste such as excrements, organic refuse and food processing dregs.    ADVANTAGE - The method improves the activity of methane bacterium stably for long period of time. The method improves the processing efficiency of activated sludge tank and methane fermentation tank simultaneously.    DETAILED DESCRIPTION - An organic waste is made into slurry and supplied in a methane fermentation tank (11). Methane fermentation of slurry is performed using anaerobic microbe, and digestive liquor is removed. The liquor is processed in an activated sludge tank (12). The organic acid concentration is measured in the tank (11). When the measured value is above predetermined value, portion of slurry containing organic acid is removed from the tank (11), solution containing organic acid separated from slurry containing organic acid is added in the tank (12) and organic acid concentration in the tank (11) is maintained under predetermined value.    An INDEPENDENT CLAIM is included for methane-fermentation processing apparatus having organic waste supply unit for slurrying organic waste and introducing the slurry in methane fermentation tank (11), organic acid detection unit (17), activated sludge tank (12) and organic acid addition unit.    DESCRIPTION OF DRAWING(S) - The figure shows the outline block diagram of methane-fermentation processing apparatus. (Drawing includes non-English language text).    Slurry adjustment tank (10)    Methane fermentation tank (11)    Activated sludge tank (12)    Separation membrane (14)    Organic acid detection unit (17)