PURPOSE: To facilitate an underwater aeration at a high efficiency, by arranging an impeller in a cylindrical hollow rotator and air injectors on the periphery of the rotator, and using a vacuum effect generated by the rotation of the rotator.

CONSTITUTION: A cylindrical hollow rotator 2 is rotated by the operation of an underwater motor 6 through a connection key 8 and a driving shaft 7. An impeller 3 in the rotator 2 gives water a centrifugal force, and sprays the water out of the rotator 2 through injection openings 5 at the top ends of hollow pipes 4 attached radially. This water injection generates a vacuum in the rotator 2. In the downstream areas at the back sides of the injection openings 5 is also generated a low vacuum, due to the cavitation effect by the rotating hollow pipes 4. The air is sucked through a center hole of the rotator 2, center holes of washers 14, 15, a center hole of a case 11 and a socket 12 by an air suction pipe 16, and injected through the air injector 4 to the surroundings. The injected air makes fine bubbles and disperses in the water. Consequently, an increased oxygen dissolving effect is obtained.