PURPOSE: To inhibit supply of an AC current to a switching power source at the time of a malfunction of an AC power source and a controller by simply suppressing a rush current at the time of ON of a power switch with a low cost.

CONSTITUTION: The overvoltage protective circuit comprises a comparator 49 for outputting a signal of a low level when a divided voltage output of a serial voltage divider of a first resistor 47 and a second resistor 48 in which a DC current rectified and smoothed from a commercial AC power source 31 flows is a reference voltage or less, a second transistor 53 to be turned ON according to the signal of the low level from the comparator 49, and a third transistor 55 to reach an ON state at the time of releasing reset of a control block. A second transistor 50, a DC terminal of an SSR 34 and the transistor 55 are connected in series, and an AC terminal of the SSR 34 is inserted to a supply line of an AC current from the power source 31 to a switching power source 35.