PURPOSE: To perform effectively a denitrification treatment while treating anaerobically sewage, by using the hydrogen sulfide generated in an anaerobic treatment stage in subjecting the sewage to denitrification reaction by autotrophic bacteria.

CONSTITUTION: The sewage contg. nitrogen components such as crude night soil or the like is charged into an acid forming tank where the sewage is subjected to acid forming reaction. The sewage is then conducted together with the return liquid mixture and return sludge from a nitrification tank to a denitrification tank where these are anaerobically mixed and agitated and are thus subjected to a denitrification treatment. The hydrogen sulfide generated from the acid forming tank is added as sulfur at about 1 mol ratio based on 1 mol nitrate nitrogen. The liquid mixture from the denitrification tank is aerobically treated in the next nitrification tank and part of the nitrified liquid is returned to the denitrification tank. The remaining liquid is discharged as treated water through settling tank.