PURPOSE: To ensure a safe reignition of burner with no risk of explosion, by providing a means adapted to interrupt the reignition of burner for a specific time duration after a miss ignition of burner.

CONSTITUTION: By opening a hot water feed cock 9, a water flow switch 9 is turned ON and a condenser C increases its voltage higher than a reference voltage. A comparator 12 outputs an operation signal to a combustion actuation circuit 11 to effectuate an ignition trial a1. In the case of ignition failure, however, a first switch circuit 16 is turned into a high-voltage state and a second switch circuit 17 is in turn maintained in a low-voltage state. An electrical discharge is thus effectuated via a second discharge circuit 14, so that the voltage condenser C may be reduced to zero. Hence, the ignition trial a1 is not actuated until the voltage of condenser C is increased to a reference voltage, even when a water flow switch 9 is turned ON in an attempt to reignite. In turn, a pre-purge a2 is actuated by natural ventilation.