PURPOSE: To prevent generation of cavitation, by raising the air drawing efficiency by locating the outlet port of a nozzle at a position closer to a discharge pipe than an air suction port.

CONSTITUTION: Water carried into a connecting pipe 23 from a submerged pump 24 is ejected into an air suction chamber 17 from an outlet port 25a of a nozzle 25 in the form of a high-speed water flow, and air is drawn from an air suction pipe 20 into the air suction chamber 17 via an air suction port 18 by the function of the high-speed water flow. Here, since the outlet port 25a of the nozzle 25 is located closer to a discharge pipe 26 than the air suction port 18 and air is drawn into the suction chamber 17 through a narrow gap by the function of the high-speed water flow, it is enabled to prevent generation of cavitation.