PURPOSE: To improve the efficiency of treatment, by reserving a liquid discharged from final treatment in a water-reserving tank, detecting BOD values in the water-reserving tank and a primary precipitation pool, and controlling the returning amount of the liquid discharged from the water-reserving tank in a manner such that the fluctuation in the BOD value in the primary precipitation pool is made small.

CONSTITUTION: In a sewage-purifying plant, aerated sewage is thickened, dehydrated and discharged as dehydrated cake at a part 7 for final treatment. At this time, water overflowing from a thickener tank 8 and liquids discharged from a digestion tank 9 and a dehydrator 11 are reserved in a water-reserving tank 12 and returned through a means 13 for controlling a flow amount to a primary precipitation pool 3. The BOD value of sewage flowing into the primary precipitation pool 3 is detected by a detector 14, the BOD value of the water reserved in the water-reserving tank 12 is detected by a detector 15, and these detected values are inputted to the means 13 for controlling a flow amount. The controller 13 controls the returning amount of the liquid discharged from the water-reserving tank 12 in a manner such that the BOD value in the primary precipitation pool 13 is made at an objective one, so that the fluctuation in the BOD value of the sewage in the primary precipitation pool 3 is made small. Thus, the stable treatment of the sewage is performed.