A monitoring system includes a plurality of monitoring devices adjacently provided to a group of integrated fuel cell stacks for measuring a voltage of the fuel cell units. Each of the monitoring devices includes a plurality of monitoring sensors adapted for electrically connecting to a predetermined set of fuel cell units and a MCU (Micro-Chip-Unit) for sequentially and circularly switching each two neighboring monitoring sensors for reading the voltage of each respective fuel cell. A CAN bus is provided for electrically and collectively interconnecting the monitoring devices with a central operator which is arranged in such a manner that when said the voltage of the respective fuel cell unit exceeds a safety range, the central operator generates an alert signal for indicating an abnormal operation of the respective fuel cell.