A battery ECU controls a main battery composed of a plurality of battery cells connected to each other in series. The battery cells are grouped into cell blocks each having a cell-block monitor circuit. A control unit controls charging/discharging processes on the basis of signals output by the cell-block monitor circuit. A signal output by a cell-block monitor circuit on a high-voltage side turns on a transistor for propagating a signal to a cell-block monitor circuit on a middle-voltage side. In the cell-block monitor circuit on a middle-voltage side, the propagated signal turns on another transistor for further propagating a signal to a cell-block monitor circuit on a low-voltage side. In the cell-block monitor circuit on a low-voltage side, the propagated signal turns on a further transistor for outputting a signal to a control unit.