PURPOSE: To lower the concentration of introduced offensive gas components and to treat the offensive gas to a level capable of discharging gas to the atmosphere by only biological oxidation reaction based on microorganisms by providing a washing water tank in the front stage for treating offensive gas in a deodorization layer and blowing out the introduced offensive gas into the washing water.

CONSTITUTION: The introduced offensive gas is uniformly dispersed into the circulated water 5 from the blowout pipe 11 of offensive gas. Thereby, the components such as ammonia and hydrogen sulfide are dissolved into liquid by Henry's law and dissociated into ions. Then, when offensive gas is introduced into a cartridge 4 housing a carrier stuck with microorganisms and brought into contact therewith, the gas is furthermore oxidatively decomposed. Finally, the treated gas is discharged into the atmosphere from the outlet 2 thereof.