An aeration unit, an aeration apparatus and a method of aeration are provided that can quickly realize water quality with gas dissolution balance appropriate for living creatures in water, and that can be utilized for treating polluted water at sewage treatment facilities and used in a gas-liquid contact step at chemical plants or others. A multiple stage aeration apparatus 6a constituted with an air diffuser 2 for producing bubbles in water, a liquid foam generating channel part 3 for changing bubbles in water into liquid foam bodies (soap-bubble-like bubble aggregate), a gas retention chamber 4 having a space capable of retaining gas in water and a bubble accumulating part 5 for collecting bubbles newly released from below the gas retention chamber 4 and feeding the bubbles to the liquid foam generating channel part 3 connected to the upper part thereof is disclosed.
