The power supply apparatus having two switching elements connected to a primary winding of a transformer includes an adjustment unit for adjusting a voltage and supplying the voltage to a control unit. This voltage results from application of a voltage induced in an auxiliary winding while current is flowing in a predetermined direction with respect to the auxiliary winding as a result of the first switching element turned on and the second switching element turned off, and a voltage induced in the auxiliary winding while current is flowing in the direction opposite to the predetermined direction with respect to the auxiliary winding as a result of the first switching element turned off and the second switching element turned on.