In an electric power supply system, a plurality of batteries (405, 406) are connected in series by a switch group (402 to 404, 407 to 409), and a higher voltage and a lower voltage are output through a terminal and a VOL terminal, respectively, and are respectively converted in the voltage thereof by two step-down DC-DC inverters (105, 106). During a discharge operation upon a serial connection, remaining content of the batteries is measured in a period other than the period of discharge from the batteries (105, 106), and the connection mode of the serial connection is controlled based on the remaining content, to control the discharge of the respective batteries up to the discharge capacity.