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.