To solve a problem that in a conventional air bubble generator, as air and hot water sucked by means of a pump are made to be finely divided and mixed by means of a high pressure jet nozzle, a high pressure and large pump is required and in addition, as a shuttle valve is used at an air suction part, it is impossible to generate stable fine air bubbles and as a two phase flow of the air and the hot water flows in the pump, life of the pump is suddenly decreased.

A fine air bubble generator with a constitution in which a gas-liquid dissolving tank 6 and a fine air bubble generating means 4 are provided in an outlet pipe 5 of a circulation pump 2 and a gas-liquid mixing part 7 is provided in an inlet part of the gas-liquid dissolving tank 6 and gas in the gas-liquid dissolving tank 6 and a gas-liquid mixed fluid flowing into the gas-liquid dissolving tank 6 are mixed and dissolved is provided and it is possible to provide the fine air bubble generator by which a gas can be dissolved into a fluid under a low pressure and in a short time and fine air bubbles can be stably generated and which uses a small-sized pump and has a simple constitution, stable performance and high reliability.