The power supply apparatus includes a control unit to control an operation of a first switching element based on a feedback voltage and a voltage of an auxiliary winding of a transformer detected by a voltage detection unit. In a state where the voltage detected by the voltage detection unit is equal to or more than a first threshold voltage, the control unit shifts to a first state where the first switching element is controlled so that the output voltage is to be a first output voltage based on the feedback voltage. In a state where the voltage detected by the voltage detection unit is less than a second threshold voltage smaller than the first threshold voltage, the control unit shifts to a second state where the first switching element is controlled so that the output voltage is to be a second output voltage smaller than the first output voltage.