Abstract: Conventionally, a non-conductive period is provided in a region in the vicinity of zero of an AC power voltage via a voltage detection circuit 14 to turn off reliably. The FET 17 maintains the ON state within several switching cycles after the non-conductive interval to rapidly restore the magnetizing coil current, so that the magnetizing coil current rapidly increases. An object is to suppress beat noise in the electromagnetic device. Within a prescribed interval following the non-conductive interval, a partial voltage at a resistor 19 of an output V2 of a mono-stable circuit 20 is added as a bias voltage to a detection voltage of a magnetizing coil current at a resistor 18, and is detected by the IC 11. The IC 11 drives a FET 17 with a constant switching period after the non-conductive interval, thereby preventing the increase in the magnetizing coil current and resolving the problem.