NOVELTY - Methane fermentation of organic waste, involves adding organic waste into a methane fermentation apparatus, fermenting the organic waste, using first sludge concentration measurement unit for measuring sludge concentration of a fermented liquid in methane fermentation apparatus, using second sludge concentration measurement unit for measuring sludge concentration of a predetermined amount of a sludge sediment liquid formed by a gravity sedimentation unit and comparing the measured sludge concentrations.    USE - The method and apparatus are useful for methane fermentation of organic waste (claimed).    ADVANTAGE - The method and apparatus can maintain a high decomposition rate in a short time without having a complex device configuration.    DETAILED DESCRIPTION - Methane fermentation of organic waste, involves adding organic waste into a methane fermentation apparatus (2), fermenting the organic waste, using first sludge concentration measurement unit (10) for measuring the sludge concentration of a fermented liquid in methane fermentation apparatus, using second sludge concentration measurement unit (20) for measuring the sludge concentration of a predetermined amount of a sludge sediment liquid formed by a gravity sedimentation unit (3), and comparing the sludge concentration measured by the first sludge concentration measurement unit and the sludge concentration measured by the second sludge concentration measurement unit using a control device (100) and controlling the amounts of liquid such that the sludge concentration of the liquid returned to the methane fermentation apparatus through a return line is higher than the sludge concentration of the fermented liquid in the methane fermentation apparatus, and the sludge concentration of the liquid discharged through a discharge line is lower than the sludge concentration of the fermented liquid in the methane fermentation apparatus. The high concentration sludge liquid at lower layer of the sludge sedimentation liquid is returned to the methane-fermentation apparatus directly or indirectly through a return line. The low concentration sludge liquid at upper layer of the sludge sedimentation liquid is discharged out of the system through a discharge line. The total amount of the liquid quantity returned through the return line and the liquid quantity discharged through the discharge line controls to be substantially equal with the quantity of the fermented liquor taken out from the methane-fermentation apparatus. An INDEPENDENT CLAIM is included for methane fermentation apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic block diagram of methane fermentation apparatus. (Drawing includes non-English language text)    Methane fermentation apparatus (2)    Gravity sedimentation unit (3)    First sludge concentration measurement unit (10)    Second sludge concentration measurement unit (20)    Control device (100)