a switching power source device for supplying power to a load includes a series resonant circuit , a plurality of main switch elements or main switch element groups for switching a current . path of the series resonant circuit , a transformer for inducing a secondary current from the series resonant circuit , a plurality of synchronous rectification switch elements for rectifying the secondary current , a maximum on width control circuit for ordering a start and a completion of a maximum on width to the synchronous rectification switch element in synchronization with a timing of turning on the main switch elements or the main switch element groups , and a synchronous control circuit . the circuit controls an on period of the synchronous rectification switch element so as to turn on the synchronous rectification switch element in synchronization with a particular timing , and turn off in synchronization with another timing .