PURPOSE: To suppress the generation of the surge voltage by switching to a minimum value by providing a switching element control circuit which controls opening and closing of a switching element of a DC/AC conversion circuit in synchronization with an OFF period of time of a switching element for a chopper.

CONSTITUTION: A PWM circuit 19 outputs signals s1 and s2 with the time width determined by the difference between the output Vr of an output current setter 17 and the output Vf of an output current detector. An output signal s5 of a chopper control circuit 21 falls at the last time of this signal s1 and the switching element 14 is cut off and a current thus decreases rapidly and comes near zero. A signal delay circuit 20a starts the time limit from this time and a signal s3 is turned OFF at the time after the set time limit. Consequently, although the switching element 4 is cut off, the current has already been diminished to zero by this time, hence the switching element 4 is cut off in an almost no-load state.