In a power supply apparatus, a power supply circuit includes a primary power supply circuit including a high-voltage battery and a step-down circuit for stepping down the voltage of the high-voltage battery, and a secondary power supply circuit including a low-voltage battery and a step-up circuit for stepping up the voltage of the low-voltage battery. The primary power supply circuit and the secondary power supply circuit are connected in parallel. An output voltage of the secondary power supply circuit is set to be lower than an output voltage of the primary power supply circuit. When the output voltage of the primary power supply circuit becomes lower than a target voltage of the secondary power supply circuit, the voltage stepped-up by the step-up circuit is supplied to a motor drive circuit. Accordingly, power supply backup of an electric power steering apparatus can be performed at low cost.