A device for controlling a dissolved amount of gas. The device is formed of: a pressure reducing part constructed by connecting a hole provided at an upper part of a closed tank to a vacuum pump through a pipe; an injection supply part for injecting fluid to be treated pressurized by a pressurizing pump from the upper part of the tank; a liquid foam generating container where the injected fluid to be treated is received, then gas of the reduced pressure space is involved in, and an air bubble group rises along an inner peripheral wall and is separated from the liquid, and then changes into liquid bubbles in an upper region; a recovery pump part for allowing the liquid bubbles having overflowed from the container; and a tank water level sensor part for maintaining a water level of the treated fluid.