PURPOSE: To set a control system gain at an optimum condition by detecting a drop of a load voltage to judge that switching from no-load to load application has been conducted.

CONSTITUTION: A load voltage is detected by means of a voltage detector 3 and inputted to a load switching judgement circuit 10, which judges switching from no-load to load application during operation based on a drop of a load voltage. A signal switching switch 12 is switched according to a load state determined by the load switching judgement circuit 10. It is switched to a no- load state gain 13 in the case of a no-load state, while switched to a load state gain 14 in the case of a load state. A determined control gain is inputted to a control gain correction circuit 11. With this constitution, the control gain can be set at an optimum condition even when the no-load is switched to the load application during operation.