PURPOSE: To prevent overshoot or undershoot from occuring at the time of release of hot water in the second time by controlling variably an ignition time responding with an opening degree of a hot water supply faucet and a required quantity of heat when using how water intermittently.

CONSTITUTION: When the opening of a hot water supply faucet is judged based on the outlet of a water current switch 22, the operation comes to a prepurge stage after the rotary motion of a fan 38. The operation is divided into two stages I which comprises a prepurge stage (0.5sec) and a prepurge II (variable) and the ignition time of a gas burner 20 from opening the faucet is variably controlled. In the prepurge stage II, a specified operation time and a stand by time are determined based on a corrected value. After the stand by time has elapsed, a gas proportional valve 36 is opened, thereby igniting the gas burner. Then, proportional control between the gas burner 20 and the fan 38 is started where the outlet temperature detected with and an outlet hot water thermistor 28 is compared with a setting temperature, thereby controlling a gas supply quantity to the gas burner 20 and an air supply quantity to the fan 38.