PURPOSE: To decrease the overshooting of hot water temperature at the time of starting the burning when the load is low by starting a slow ignition with the switch valve held in the opened position if the load, i.e., the required calorific value to bring the hot water temperature up to the set temperature is high, and in the closed position if such a load is low.

CONSTITUTION: By carrying out a slow ignition with the switch valve held in the closed position if the load determined by an FF computation means 10b is low, the calorific value is made smaller than that which is generated when the switch valve is opened so that the overshooting of hot water temperature may be decreased at the time of ignition. After determining the load level for the optimum switching point of the switch valve 8, a slow ignition is carried out with the switch valve held in the opened position if the load level determined by the FF computation means 10b is high, and in the closed position if the load level is low. This way, the degree of overshooting can be decreased at the time of ignition if the load level is low.