PURPOSE: To prevent infeasibility of communication between remaining ECRs due to the fault of a specific ECR set as a master station from being generated, by operating each ECR as the master station, and operating the ECRs as the master station at a prescribed order.

CONSTITUTION: At the time of assuming that the minimum time is set at a first timer circuit 7 in the ECRR1, the first timer circuit 7 in the ECRR1 outputs a first time out signal. Thereby, the ECRR1 detects the time out signal, and is operated as the master station, and sends certain data to a transmission path L. Then, the remaining ECRs R2∼Rn receive the data, respectively, and reset the first timer circuits 7. Also, when the ECRR1 is not operated as the master station due to an unknown reason and no data is sent to the transmission path L, the ECR in which the next minimum time is set in its first timer circuit 7 is operated as the master station. Thus, either ECRs R3∼Rn functions as the master station, thereby, it is possible to prevent the communication of the whole system from being infeasible.