PURPOSE: To save consumption of desulphurizing chemical and to remove H2S with high efficiency by allowing generated gas from a methane fermentation tank to react catalytically with waste water contg. SO2 and converting H2S in the generated gas to simple substance of sulphur.

CONSTITUTION: Original waste liquid is led to a fermentation tank 1 through a line 11, and digested liquid obtd. in the digestion tank by the methane fermentation treatment is discharged through a line 12. Generated gas is led through a line 15 into a desulphurization tower 3 where it is allowed to react catalytically with waste liquid contg. SO2 introduced from line 16; H2S in the generated gas is converted to simple substance of sulphur. Thus, H2S in the generated gas is removed. Desulphurized generated gas is introduced into a gas holder 4 through a line 18. A part of the gas is taken out through a line 19 and a boiler 5 and is utilized for energy source, and the other part of the gas is returned to the fermentation tank 1 through a line 20, boiler 6, and a line 14, to be utilized for stirring of the tank.