A communication network for use in an automobile includes a main electronic control unit (ECU) that communicates with plural sub-ECUs connected to the main ECU through a communication line. Messages transmitted periodically with a certain interval from each sub-ECU to the main ECU and received by the main ECU in a monitoring interval are counted to obtain the number of message receipts. The number of message receipts is compared with a criterion number preset for each sub-ECU, and if the former is smaller than the later, it is determined that the sub-ECU is under failure. Further, if the number of message receipts is smaller than the criterion number as to all of the sub-ECUs, it is determined that there is a failure or failures in the communication line.