An electric power steering device includes a main power supply unit capable of supplying power to a load; an auxiliary power supply unit connected to the main power supply unit and capable of supplying charged power to the load; a boosting circuit capable of boosting a voltage supplied from the main power supply unit and applying a resulting voltage to the auxiliary power supply unit; a circuit switching unit interposed between the main power supply unit and the load and closing one of a first connection circuit extending from the main power supply unit to the load and a second connection circuit extending from the main power supply unit to the load via the auxiliary power supply unit; and a controller performing switching control on the circuit switching unit so as to close the second connection circuit if the magnitude of power required by the load exceeds a first threshold.