The controller controls a cell selection circuit to electrically connect both ends of a cell to be discharged among n cells connected in series and both ends of the inductor for a predetermined time. Next, the controller controls the cell selection circuit to electrically cut off the n cells and the inductor, and turn on the clamp switch. Next, the controller turns off the clamp switch, and controls the cell selection circuit to electrically connect both ends of a cell to be charged among the n cells and both ends of the inductor for a predetermined time.