A power supply of a vehicle includes a first switching element, a second switching element, a third switching element, a first battery, a reactor element, a second battery, a smoothing capacitor, and a controller. When the connection state of the first battery and the second battery is switched from the series connection state to the parallel connection state, the controller is configured to perform a transition control so that a voltage of the smoothing capacitor is decreased to the higher of a voltage of the first battery and a voltage of the second battery, and perform a switching control to turn on the first switching element after a diode of the first switching element is energized.