PURPOSE: To enable an efficient aeration treatment without requiring diluting water and to provide reduced running costs, the reduced size and driving power of the tank, etc., by providing a weak aerator to one part of the aeration tank and a strong aerator to the other part and circulating waste water between both parts.

CONSTITUTION: Waste water which is the original liquid to be admitted into an aeration tank 1 from an arrow (a) direction is maintained at ≤0.3ppm average concn. of dissolved oxygen by a weak aerator 2 in the lower layer part. Said water is maintained at the average concn. of dissolved oxygen higher by ≥0.1 ppm than the lower layer part by a strong aerator 3 in the upper layer part. The waste water in the upper layer part and the waste water in the lower layer part are mixed by a circulative moving device for waste water, by which fresh sludge is formed and is discharged in an arrow (b) direction. The water is separated to cleaned waste water and sludge in the next separator.