At either of the two terminals of each of a plurality of cells constituting a battery pack, a detection terminal is provided, and malfunction detection circuits each provided in correspondence to one of the cells each detect a malfunction of the corresponding cell based upon the voltage between the detection terminals. When shorting circuits are engaged in operation to short detection terminals corresponding to alternate cells from each other, a disconnection having occurred between a given cell and a corresponding detection terminal is detected based upon signals output from the malfunction detection circuits.