Abstract: An aerator comprises an air diffuser rotor for receiving air and forcing out the air through small perforations formed in its peripheral wall, and an agitator-pump unit rotatably disposed immediately above the rotor. When the aerator is installed on the bottom of a waste water purifying tank, the agitator-pump unit raises water from the bottom of the tank and produces a circulating flow of water through the interior of the tank, while the air diffuser rotor forces out air bubbles into the circulating flow, whereby an improved aeration efficiency can be achieved.