A boost circuit is connected to a rechargeable battery and a switch is connected between a rectification circuit and a power factor correction converter. In the power failure state, the switch is turned on to activate the boost circuit and energy of the rechargeable battery is supplied to a load by rasing the voltage at two steps in the path from the boost circuit, switch, and to power factor correction converter. Since the voltage of the battery can be lowered, the number of serially connected cells can be made small. The volume of UPS can be reduced and a low cost can be realized. Since the number of serially connected cells is reduced, the reliability against failure of battery cells can be improved.