To provide a structure of a gas-liquid dissolving tank with which a water current containing large bubbles is prevented from flowing out of an outlet of the tank, the amount of minute bubbles to be generated is increased by improving the gas-liquid agitating capacity, a cavitation is prevented from occurring in a pump and the malfunction of the pump caused by air involvement is prevented.

This gas-liquid dissolving tank has such constitution that a tank part 2 arranged on a backward route of the pump 1 is formed into a horizontal shape by combining three or more cylindrical bodies different in diameter in layers. A nozzle-shaped ejector part 9 the cross-sectional area of which is throttled for jetting water and air is faced to a central flow passage 3a of the central cylindrical body 3 which is positioned in the innermost layer and has the smallest diameter. An intermediate flow passage 4a to be communicated with the central flow passage 3a is formed between the central cylindrical body 3 and an intermediate cylindrical body 4. An outer peripheral flow passage 5a is formed between the intermediate cylindrical body 4 and an outside cylindrical body 5, one end of which is communicated with the intermediate flow passage 4a and the other end communicates with the tank outlet 6 arranged on the backward route in the tank part 2 and an air discharge port 7. A surplus air storage part 8 is formed in the upper layer part of the intermediate flow passage 4a for storing surplus air A.