PURPOSE: To reduce the load of a transformer and to reduce a drop in a forward voltage by suppressing the flow of the backward current of a power MOSFET for synchronous rectification.

CONSTITUTION: The controller of a power MOSFET 13 for synchronous rectification is connected to the side of a secondary winding 1S for a pulse transformer 1 in a switching power-supply apparatus which uses a flyback converter circuit, a forward converter circuit or the like as a fundamental circuit, and it rectifies an output current. Thereby, for example, comparator circuit 14 is provided as a backward-current prevention means wherein a change in the direction of a current flowing across a source and a drain for the power MOSFET 13, for synchronous rectification, which can control a forward current and a backward current by a gate driving signal is monitored and a driving signal which is supplied to a gate for the power MOSFET for synchronous rectification is stopped while the backward current flows.