PURPOSE: To secure the transmitting function for the fault of a transmission system as well without duplexing a transmission line and to simplify the configuration of the transmission line.

CONSTITUTION: Respective nodes 11-15 are provided with two interfaces I/F for LAN and provided with functions for disconnecting and connecting both the interfaces, a transmission cable 16 is spread between the respective nodes in the manner of a loop, provided with a terminating resistor TM and a transceiver unit TU at each node position and coupled with one interface of any relevant node, at normal time, the function of transmission between the respective nodes is secured by disconnecting both the interfaces at one node and at the time of the fault of the transmission system such as the disconnection of the transmission cable, the function of transmission between the respective nodes is secured by disconnecting both the interfaces at a certain node to be connected to the relevant fault part and connecting both the interfaces at the other node.