A plurality of batteries connected in series are monitored by a plurality of monitor devices, respectively. The monitor devices are connected in series to communicate under the daisy chain communication system. The daisy chain communication system includes a micro-control unit for producing a binary signal, a communication device for converting the binary signal to a pulsating signal. The communication device produces a special code based on an instructions from the micro-control unit. Each monitor device changes between a communication OFF state and a communication ON state. Each monitor device has a signal detector which operates during the communication OFF state and detects the special code, whereby upon detection of the special code, the signal detector generates a wakeup signal so that the monitor device is changed from the communication OFF state to communication ON state.