To control a temperature of hot water of a gas hot water boiler in a positive manner to attain its constant temperature and further to perform a fine adjustment of a gas flow rate.

A thermo-sensitive operating member 80 contacts with water in a hot water pipe 41 of a gas hot water boiler to each other. The thermo- sensitive operating member 80 has an expanding or retracting head section 82 and the expanding or retracting head section is expanded or retracted in response to a variation of water temperature. A piston 70 is cooperatively connected to the expanding or retracting head section 82 of the thermo-sensitive operating member 80, thereby it is operated in concurrent with the expanding or retracting head section 82 and then an aeration pipe port is controlled in response to motion of the piston 70, thereby a flow rate of gas is adjusted and its temperature is constantly controlled.