PURPOSE: To treat any conc. viscous organic waste matter as such and to accelerate and stabilize the formation of gas, by holding an upper space in a digesting cell at fixed negative pressure to manage the space under a decompressed condition, and letting gas formed by digestion (fermentation) flow through organic waste matter to agitate the waste matter.

CONSTITUTION: On the basis of the detected 8 pressure of a space inside a tank 4 closed with a cover 6, pumps 22, 26 and valves 24, 39 are ON-OFF controlled by controlling parts T, P. By this operation, CH4 gas formed in the space 7 is returned to the tank 4 through each of the circulating lines of a withdrawing opening 15- an outflow pipe 16- a desulfurizer 17- the pump 22- the valve 24- a check valve 25- a discharging opening 20 and the pump 26- a check valve 27- a discharging opening 21, to agitate excrementitious matter and to manage the space 7 under a condition decompressed to fixed negative pressure. An atmospheric temperature in the tank 4 is raised and held t a value suited to fermentation by a thermometer 9 and a hot water pipe 31 to be controlled. As a result, the excrementitious matter in the tank 4 is digested an fermented under the optimum condition, and the formation of gas is facilitated by evacuation and agitation.