PURPOSE: To always keep activity of carrier without deterioration by charging plural balls in an air-separating part of air lift pump provided in an aeration tank.

CONSTITUTION: The air life pump 20 having air-separating part 20C charged with plural balls 21 is provided in the aeration tank 8. That is, the carrier contained in the aeration tank is carried to the air-separating part 20C in the upper part of the pump 20 together with water. As the balls 21 are charged in the air-separating part, the excess activated sludge is peeled off and removed from the surface of the carrier by allowing the surface of the carrier to contact with the balls 21. Thus the carrier always maintains a suitable size and deterioration of activity is prevented.