The present invention is to provide a voltage detector having a common communication line for reducing a manufacturing cost. A low voltage line CPU transmits a detection instruction including an assignment of one address among a plurality of blocks. The detection instruction is branched by a transmitting bus line for concurrently transmitting to a plurality of voltage detector units. When one of the voltage detector units receives the detection instruction with the address being same as a self-address thereof, the voltage detector unit detects output voltages of unit cells and transmits the detected result to the low voltage line CPU. When the received address is not the self-address, the voltage detector unit does not transmit the detected result.