PURPOSE: To realize the ratinalization of the switching process and at the same time prevent the confusion of the information transmission at the switching time, by transmitting the switch command also to the remote station and then performing the switching by selecting an adequate time point at the both stations.

CONSTITUTION: In case some fault occurs at the circuit 2-3 with a reduction given to the level Sl of the transmission signal supplied to the receiving device 8 at the station 1, the monitor device 9 delivers the signal Sd. This signal Sd is supplied to the control device 6 of the station 1 via the bus 5, and the device 6 delivers the switch command Sw to the bus 5. Then one of the command Sw is supplied to the device 6 via the transmitting device 4, the switch 3, the circuit 2-1, the switch 7 of the station 2, the receiving device 8 and the bus 5 each; and the other command is supplied to the switching device 10 of the station 1. On the other hand, the switch signal Sw of the station 2 is delivered also through the device 6 of the station 2 to the device 10 via the bus 5. Here the devices 10 of the stations 1 and 2 switches the switches 3 and 7 to the position opposite to those shown in the figure, and accordingly the subsequent transmissions are carried out via the circuits 2-2 and 2-4.