A monitoring device comprising: an assembled resistance composed of two or more resistances (131, 132, 133; 141, 142, 143) connected in series, configured to have one end connected to a positive terminal of a storage battery (101; 102) connected in series and the other end connected to a negative terminal of the same storage battery (101; 102), and determining a midpoint potential higher than the negative terminal potential and lower than the positive terminal potential; a processing means (115; 125) for detecting a status of the storage battery (101; 102); and two or more communication means (111; 121) at respective negative terminal potentials of the plurality of storage batteries for enabling the processing means (115; 125) to communicate with the processing means (115, 125) of another monitoring device or for enabling the processing means to communicate with an exterior controller; characterized by further comprising signal isolation means (116, 117; 126, 127) for obtaining a signal isolation in the communication between the processing means (115; 125) or the communication with said exterior controller; wherein the processing means (115; 125) connected to the midpoint potentials determined by the assembled resistances, and: • have a current detection means for detecting the current flowing in the assembled resistance, or • have a voltage detection means for detecting the voltage of a portion of the assembled resistances, wherein the current detection means or the voltage detection means are configured to determine a failure of the signal isolation means (116, 117, 126, 127) using the detected current or the detected voltage. an assembled resistance composed of two or more resistances (131, 132, 133; 141, 142, 143) connected in series, configured to have one end connected to a positive terminal of a storage battery (101; 102) connected in series and the other end connected to a negative terminal of the same storage battery (101; 102), and determining a midpoint potential higher than the negative terminal potential and lower than the positive terminal potential; a processing means (115; 125) for detecting a status of the storage battery (101; 102); and two or more communication means (111; 121) at respective negative terminal potentials of the plurality of storage batteries for enabling the processing means (115; 125) to communicate with the processing means (115, 125) of another monitoring device or for enabling the processing means to communicate with an exterior controller; characterized by further comprising signal isolation means (116, 117; 126, 127) for obtaining a signal isolation in the communication between the processing means (115; 125) or the communication with said exterior controller; wherein the processing means (115; 125) connected to the midpoint potentials determined by the assembled resistances, and: • have a current detection means for detecting the current flowing in the assembled resistance, or • have a voltage detection means for detecting the voltage of a portion of the assembled resistances, • have a current detection means for detecting the current flowing in the assembled resistance, • have a voltage detection means for detecting the voltage of a portion of the assembled resistances, wherein the current detection means or the voltage detection means are configured to determine a failure of the signal isolation means (116, 117, 126, 127) using the detected current or the detected voltage.