A switching power source device which has main switch elements which switch a current path of the series resonant circuit, and a transformer which induces a current to a secondary side, controls the main switch elements on a primary side. Synchronous rectification switch elements are turned ON and OFF in response to one of the main switch elements. A synchronous control circuit which turns the synchronous rectification switch element ON in synchronization with an ON timing of the main switch element, or a conduction timing of internal diodes in the synchronous rectification switch elements detected by an inter-terminal voltage signal of the synchronous rectification switch element, whichever timing is later, determines a maximum ON width of the synchronous rectification switch element in accordance with a delay time of the conduction timing of the internal diodes with respect to the ON timing of the main switch element.