A power source apparatus comprises: a transformer that insulates a primary system and a secondary system and uses primary/secondary windings to transform an input voltage into an output voltage; a switching control device that is disposed in the primary system to drive the primary winding, and an output monitor device that is disposed in the secondary system to monitor the output voltage. The transformer includes a first auxiliary winding disposed in the primary system and a second auxiliary winding disposed in the secondary system. The output monitor device drives the second auxiliary winding to generate an induced voltage in the first auxiliary winding when the output voltage becomes smaller than a predetermined threshold voltage. The switching control device temporarily stops driving of the first winding upon detecting a light load state and resumes the driving of the first winding upon detecting the induced voltage in the first auxiliary winding.