PURPOSE: To supply fine bubbles to waste water with high efficiency in the treating device for domestic waste water, etc., by forming a rotary aerator with a cylinder with a spiral plate fixed to the inner wall and the gas inlets arranged directly below the lower opening of the cylinder at intervals and suspending the aerator at the bottom of a waste water treating tank.

CONSTITUTION: An aerator 30 is suspended at the bottom of a contact tank 53 for bringing waste water into contact with a contact material 54 and biochemically oxidizing the waste water. This aerator 30 is formed as the rotary aerator with a cylinder 60 with plural spiral plates 61 having lead angle of 0.7 to 1.3 and fixed to the inside and the air inlets 66 arranged at intervals at the lower opening of the cylinder 60. The lower opening of the cylinder 60 is contracted to form a gas and liq. inlet 63. As a result, the air in the gas feed pipe 65 is released into the contact tank 53 from the inlets 66, introduced into the cylinder 60 from the inlet 63, spiraled along the plate 61 and sent upward. The air is micronized by the shearing force of the spiraling waste water and discharged into the waste water from the upper part of the cylinder 60.