The power supply apparatus includes a rectifying and smoothing unit configured to rectify and smooth an AC voltage of an AC power supply, a transformer having a primary winding, first and second secondary windings, a detecting unit connected to the second secondary winding, the detecting unit configured to detect a second voltage according to a voltage output from the rectifying and smoothing unit, and a first control unit configured to control a state of the load based on a result detected by the detecting unit.