PURPOSE: To perform effectively methane fermentation at the specified stagnation time of a raw material in a pretreatment tank of a methane fermentation device and rate of supply of the raw material to the fermentation tank by forming the pretreatment tank in a hermetic vessel and discharging the raw material from the same by using a pump for supplying the raw material.

CONSTITUTION: A raw material such as sewer sludge, night soil or the like is diluted in a mixing tank 40 and is prepared into a slurry. The slurry-like raw material is charged continuously into a pretreatment tank 60. Since the tank 60 is formed of a hermetic vessel, the amt. of the raw material equal to the discharged amt. of the raw material is forced out by the discharging pressure of the pump 64 and is admitted into a fermentation tank 74. If the discharge of the pump 64 is kept constant during this time, the amt. of the raw material flowed out of the tank 60 into the tank 74 and the stagnation time of the raw material in the tank 60 are maintained constant and the effective pretreatment is accomplished.