PURPOSE: To realize low cost through shortened release time and simplified circuit structure by connecting in series a flywheel diode and capacitor and eliminating a switching transistor.

CONSTITUTION: A series circuit of flywheel diode 4 and capacitor 7 is connected in parallel to an electromagnet coil 3, and a parallel circuit of collector-emitter of transistor 2 and a voltage suppressing element 5 is connected in series to said electromagnet coil 3. As a current of capacitor 7, a discharge current id flows when the transistor 2 is ON, while a charge current ic flows, owing to a reverse electromotive force when the transistor 2 is OFF. In case the chopper stops because an input voltage is turned OFF, a capacitor 7 has a high impedance and a reverse electromotive force commutates automatically to the voltage suppressing element 5 and a release time is shortened.