A voltage converting unit converts a first voltage of an auxiliary power supply unit into a second voltage. A voltage determining unit determines whether the second voltage exceeds a predetermined threshold. A first switch, which is arranged between the voltage converting unit and a load, switches on and off a supply of an electric power of the second voltage to the load. When it is determined that the second voltage exceeds the predetermined threshold, a voltage-conversion control unit controls the voltage converting unit to stop converting the voltage, and then a switching control unit controls the first switch to supply the electric power of the second voltage to the load.