A power supply apparatus of a vehicle includes: an engine and a first MG; a battery; a converter stepping up a voltage of the battery and supplying the stepped-up voltage to an inverter of the vehicle; and a control device controlling the converter in a continuous voltage step-up mode in which the converter is continuously operated and an intermittent voltage step-up mode in which the converter is intermittently operated. The control device estimates an SOC of the battery based on battery current IB flowing into and out of the battery, and forces the battery to be charged by the engine and the first MG when an estimate value of the SOC is lower than a predetermined lower limit. The control device suppresses an operation of the converter in the intermittent voltage step-up mode to a greater extent as the estimate value of the SOC is closer to the lower limit.