An auxiliary power supply device includes a capacitor that is connected to a main power source, and that is able to discharge an electric current to an electric motor, the main power source supplying electric power to the electric motor； and a booster circuit that boosts a voltage of the main power source and applies the boosted voltage to the auxiliary power source. An operation of the auxiliary power supply device is controlled by a control device. When supply of the electric power from the main power source to the capacitor is started in a state where a voltage across terminals of the capacitor is equal to or higher than a voltage of the main power source, the control device increases the voltage applied to the capacitor by the booster circuit as the capacitor voltage increases.
