A voltage monitoring apparatus for a plural battery, the plural battery comprising a plurality of cells (BT1-BT5) connected in series and being configured to output a required voltage and the apparatus being configured to monitor an abnormality in an output voltage of each of the cells, the apparatus comprising: a voltage abnormality detector (21-1,21-2,21-n) provided for each of a plurality of blocks into which the plurality of cells (BT1-BT5) are divided, the voltage abnormality detector is configured to detect a voltage abnormality in each of the cells contained in the block, the voltage abnormality detector including: a voltage detector configured to detect a cell voltage of each of the cells contained in the block; a comparator configured to compare the cell voltage detected by the voltage detector with preset upper and lower thresholds and determine whether or not the cell voltage is overcharge or overdischarge; and a transmitter (57,58) configured to transmit the determination result of the comparator to the monitoring unit by way of a digital determination signal and an analog determination signal; and a monitoring unit configured to determine whether or not a communication error is occurring in a communication line connecting the voltage abnormality detector (21-1,21-2,...) and the monitoring unit to each other, according to the digital determination signal and analog determination signal wherein the monitoring unit is configured to confirm whether information about overcharge and overdischarge of the cells derived from the digital determination signal agrees or disagrees with information about overcharge and overdischarge of the cells derived from the analog determination signal, and based on the confirmation, to determine whether or not a communication error is occurring in a communication line between the voltage abnormality detector and the monitoring unit. a voltage abnormality detector (21-1,21-2,21-n) provided for each of a plurality of blocks into which the plurality of cells (BT1-BT5) are divided, the voltage abnormality detector is configured to detect a voltage abnormality in each of the cells contained in the block, the voltage abnormality detector including: a voltage detector configured to detect a cell voltage of each of the cells contained in the block; a comparator configured to compare the cell voltage detected by the voltage detector with preset upper and lower thresholds and determine whether or not the cell voltage is overcharge or overdischarge; and a transmitter (57,58) configured to transmit the determination result of the comparator to the monitoring unit by way of a digital determination signal and an analog determination signal; and a monitoring unit configured to determine whether or not a communication error is occurring in a communication line connecting the voltage abnormality detector (21-1,21-2,...) and the monitoring unit to each other, according to the digital determination signal and analog determination signal wherein the monitoring unit is configured to confirm whether information about overcharge and overdischarge of the cells derived from the digital determination signal agrees or disagrees with information about overcharge and overdischarge of the cells derived from the analog determination signal, and based on the confirmation, to determine whether or not a communication error is occurring in a communication line between the voltage abnormality detector and the monitoring unit. a voltage detector configured to detect a cell voltage of each of the cells contained in the block; a comparator configured to compare the cell voltage detected by the voltage detector with preset upper and lower thresholds and determine whether or not the cell voltage is overcharge or overdischarge; and a transmitter (57,58) configured to transmit the determination result of the comparator to the monitoring unit by way of a digital determination signal and an analog determination signal; and a monitoring unit configured to determine whether or not a communication error is occurring in a communication line connecting the voltage abnormality detector (21-1,21-2,...) and the monitoring unit to each other, according to the digital determination signal and analog determination signal wherein the monitoring unit is configured to confirm whether information about overcharge and overdischarge of the cells derived from the digital determination signal agrees or disagrees with information about overcharge and overdischarge of the cells derived from the analog determination signal, and based on the confirmation, to determine whether or not a communication error is occurring in a communication line between the voltage abnormality detector and the monitoring unit.