A monitor (5) monitors an assembled battery (1) having a plurality of cells (11) and a power supply line (2) for connecting the plurality of the cells (11). The monitor (5) includes: a detecting portion (6) for detecting a voltage of each of the cells (11) of the plurality of the cells (11); and a controlling portion (7) for receiving a voltage detection result detected by the detecting portion (6). The detecting portion (6) converts the voltage detection result into an alternating current communication signal and transmits the alternating current communication signal via the power supply line (2) to the controlling portion (7).