NOVELTY - A fixed bed type methane fermentation tank (15) has a filling tank (16) loaded with a porous filler to which the methane fermentation microbes are adhered, and a mixing tank (17) provided at the lower portion of the filling tank. A circulation system (19) returns the liquid of organic waste, which is extracted from the upper portion of the filling tank to the mixing tank.    USE - Fixed bed type methane fermentation system for biogas production, used to carry out methane fermentation of organic waste obtained from houses and hotels, and waste foodstuffs e.g. vegetables, obtained from foodstuffs manufacturing processes, in order to produce biogas.    ADVANTAGE - The liquid of organic waste is mixed with the sludge containing the microbes, hence the non-uniformity in the concentration of the organic waste in the tank is not produced. The microbes and organic waste are made to contact uniformly, hence improving the gas production rate. The stirring effect of the mixing tank is promoted by returning the liquid to the mixing tank, hence the methane fermentation efficiency is increased.    DETAILED DESCRIPTION - The upper and lower extraction lines (27,28) extract a liquid from the upper portion of the filling tank and from the mixing tank, respectively. The outlet valves (29,30) are provided in the upper and lower extraction lines, respectively. The flow rate of a circulation pump (31) and the opening of the valves are adjusted to adjust the linear velocity of the liquid raising inside the filling tank in the range of 0.3-1.3 m/seconds, and the liquid circulating load of the mixing tank. When the liquid circulating load of the mixing tank is set to V (m3), and liquid flow rate is set to F (m3/hour), the liquid substitution frequency N (times/hour) is given by the formula: N equal to Fdivide byV, and is set in the range of 3-6. The volume of the mixing tank is in the range of 5/100-20/100 of the volume of the filling tank.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the methane fermentation system.    methane fermentation tank (15)    filling tank (16)    mixing tank (17)    circulation system (19)    upper and lower extraction lines (27,28)    outlet valves (29,30)    pump (31)