PURPOSE: To perform methane fermentation of high concn. org. waste water in good efficiency, by recirculating and flowing waste water through a packing layer having methane producing bacteria adhered thereto at a flow speed of a specific value or more in a flow amount with a specific value or more.

CONSTITUTION: A packing layer packed with a carrier having methane producing bacteria adhered thereto is charged in an anaerobic reactor 2 and high concn. org. waste water is recirculated from the upper part of the reactor 2 to the lower part thereof through a recirculation path 3 by a pump 4. The emitting capacity of the pump 4 provided to the recirculation path 3 is set so as to flow waste water at flow speed of 50cm/min or more in a flow amount eight times or more of the supply amount of raw water and a part of the recirculated treated water is taken out from the upper part of the packing layer 1 and methane gas is also simultaneously taken out.