PURPOSE: To reduce wiring change at function replacement of a transmission reception section by providing a distributer of a signal from a subordinate transmission section and a selector for distributed outputs to the transmission section and providing a distributer for an output of a multiplexer demultiplexer section and a selector for distributed signals to the reception section.

CONSTITUTION: When a fault takes place in, e.g. an active high frequency line 5, control sections 201, 401 detect it and inform it to control sections 301, 501 of a standby high frequency line transmission section 30 and a reception section 50. The control section 301 controls a selector 2 to select a signal from a distributer 4 and sends a command to the control section 201 to allow a selector 1 of the transmission section 20 to select the position of a low frequency signal input section 3. The control section 501 allows a selector 10 to select the position of the distributer 9 and sends a command to the control section 401 and a low frequency signal output section 8 of the reception section 40 selects and receives a signal from a distributer 11 and outputs it. Then the communication is implemented by using a standby line 31 in place of an active high frequency circuit 5. In this case, the communication at a low frequency line group is tentatively stopped.