A power supply apparatus of a vehicle includes: auxiliary machines electrically connected between a battery and a converter; and a control device controlling inverters and the like based on a required vehicle power including a required charge discharge amount of the battery. The control device has a continuous voltage step-up mode and an intermittent voltage step-up mode. In the continuous voltage step-up mode, the control device calculates the required charge discharge amount based on electric power supplied from the battery. In the intermittent voltage step-up mode, the control device changes a method of calculating the required charge discharge amount to a method that calculates the required charge discharge amount based on electric power passing through the converter, and makes the required charge discharge amount smaller than the required charge discharge amount in the continuous voltage step-up mode.