PURPOSE: To droop an output voltage by sufficiently supplying a reset current of a saturable reactor even in an overload state.

CONSTITUTION: A switching element 2 is connected to a primary winding 4 of a main transformer 3, and a saturable reactor 6, a rectifier 7 and a resetter 8 are connected to a secondary winding 5. A reset current is supplied to the reactor 6 through a transistor Q1 of the resetter 8, a rectified output voltage V2 of a tertiary winding 9 is applied to an emitter of a transistor Q1 of the resetter 8 when an output voltage V1 in a load short-circuiting state is lowered, and the reset current of the reactor 6 is supplied to effectively droop the voltage V1.