PURPOSE: To improve the maintenance performance and the reliability of the system and to prevent a fault of disabled communication consecutiveness due to a fault of a standby side bus by always monitoring the standby side bus of duplicate buses and detecting the fault and locating a fault location when the fault takes place in the bus.

CONSTITUTION: An active bus L1 is interconnected to a communication section 1 and an active side bus monitor section 3 by a bus changeover section 2 according to a command of a host communication section 1, the monitor section 3 receives a signal from the bus L1 and when a start delimiter in the received frame signal is detected, it is sent to a clock section 5. However, when the signal is not token frame addressed to a communication controller, a wrong destination signal is sent to the clock section 5 to allow the standby set side bus monitor section 4 to receive the signal from the standby set side bus L2 thereby checking all received frames. When the frame is normal, the clock section 5 is started, the operation of the clock section 5 is stopped by an output from the monitor section 4. Furthermore, when a fault takes place in the standby set side bus L2 while the clock is sent, it is fed to a fault detection section 6 and informed to the communication section 1.