The battery pack includes an assembled battery including a plurality of secondary battery cells connected at least in series; a voltage detector which detects a terminal voltage of each of the plurality of secondary battery cells; a charge/discharge controller which controls charge/discharge of the assembled battery based on the terminal voltage of each cell detected by the voltage detector; a short-circuiting section which short-circuits a node between cells to the high voltage side or the low voltage side of a power line, or which short-circuits nodes between the cells to each other; and a disconnection detector which controls conduction/non-conduction of the short-circuiting section and which detects a disconnection of the connection line from the voltage detector to the node between the cells, based on a state of controlling the conduction/non-conduction, and at least one of a voltage at the node between the cells detected by the voltage detector, a voltage of the high voltage side or the low voltage side of the power line, and a terminal voltage of each cell. The floating (disconnection) of intermediate taps of the secondary battery cells can be reliably detected without causing losses in regular processes.