To solve the following problems: in an underwater aeration device of a type blowing high pressure air produced by an air compressor into water, a large-power air compressor is required for pushing heavy water away with air, and the blown air does not significantly spread in the water, so that the oxygen-dissolved water area does not become wide.

An inner cylindrical body 7 is arranged within an outer cylindrical body 6, and a bent water-intake pipe 8 is coupled to the inner cylindrical body 7. One end of an air-intake part 10 is opened and the other end is blocked. The open end is connected to the middle of the water-intake pipe 8, and a pump water jet pipe 9 is hermetically arranged through the blocked end. The tip of the pump water jet pipe 9 is located within the water-intake pipe 8, and a pump water supply pipe 11 is connected to the rear end thereof and supplies a high-pressure water stream from a pump 12. Air on the water surface is taken into the air intake part 10 through an air-intake pipe 4. An air-mixed water stream discharged from the outer cylindrical body 6 spreads widely in the water.