To quicken the rising of a reference voltage, and to improve a tracking property to a target frequency by varying a power supply voltage for varying the amount of blast of a blast fan by the amount of change per unit time until a target voltage is reached by a DC motor for reaching the target voltage.

It is determined that a fan-motor 4 is abnormal according to a result obtained by comparing the difference to a target value of a drive circuit current being detected by a current detection circuit 6 or a reference voltage and a speed detection circuit 8, thus reducing a voltage or the like, In this case, the voltage of a reference voltage generation circuit 1 is controlled to be changed, unit a target speed is reached. For example, voltage on a safer side is allowed to cope with a case where it is determined that the motor is abnormal, since the voltage being applied to a circuit during activation does not reach the target voltage, thus increasing or decreasing the power voltage for the drive part of a DC motor for driving the blast fan for a hot water supply machine so that the power voltage is changed in time series in steps, quickening the rising of the reference voltage, and improving a tracking property to the target speed.