PURPOSE: To facilitate the switching of the anaerobic to aerating operation by providing a suction port at the upper part of a hollow shaft and a discharge port almost at the center and furnishing a screw respectively at the lower end of the shaft and at the center slightly above the discharge port.

CONSTITUTION: The lower screw 23 and upper screw 24 are simultaneously rotated by a motor 21 through a hollow shaft. The upper screw 24 and a discharge port 26 bored below the upper screw 24 are placed below the water surface when the hollow shaft 22 is inserted (approached) into the water at a specified angle and aeration is carried out, and the upper screw 24 and discharge port 26 are placed above the water surface at an appropriate position of the center of the shaft when the approach angle is made larger than that in aeration and anaerobic operation is carried out. As a result, a valve for sucking air into the aerator need not be provided, and the aerating and anaerobic agitated operations are easily selected only by adjusting the approach angle of the shaft to the water surface.