To provide the aerator by which the residence time of the superfine air bubble in the water is prolonged, the oxygen dissolving efficiency is improved, and aeration is efficiently performed in the wide water areas.

Plural cylindrical nozzle members 1, 2 and 3 with a water passage formed inside are arranged on the same axis, ring slit-shaped suction nozzles 6 and 7 are formed between the respective members, an outer casing 5 is provided to cover the periphery of the nozzle members 1, 2 and 3, a high- pressure water supply pipe P1 for supplying high-pressure water is connected to one end of the nozzle member and an air feed pipe P2 for feeding air to the nozzles 6 and 7 to the outer casing 5, a tank 9 is furnished to supply a large amt. of water to the periphery of the tip of the final nozzle member 3, and a discharge pipe 4 is connected to the outer casing so that the water from the tank is discharged to surround the periphery of the high-pressure air-water mixture current discharged from the final nozzle member 3.