PURPOSE: To improve the reliability of the protection network of a synchronous switching power supply, in which the switching power supply is controlled in driving by a feedback pulse.

CONSTITUTION: An apparatus is constituted by a switching power supply 2 for generating a certain voltage with the use of an incoming synchronizing pulse as trigger, a flyback transformer 3 and low-voltage transformer 3 to which a predetermined synchronizing signal is supplied and a certain voltage from the switching power supply 2 is applied and which then output an output voltage synchronized with the synchronizing signal and supply the synchronizing pulse to the switching power supply 2, an overcurrent detector 5 for detecting the value of output current from the switching power supply 2 and a power- stopping circuit 6 for stopping the switching power supply 2 when the value of the output current exceeds a predetermined value.