Abstract: Disclosed is a duplex system of transmission lines in a loop network applied, for example, to a local area network. To realize an autonomous recognition of transmission line faults by each communication node and to enable a rapid and automatic reconstruction of a network, each communication node comprises a loop back circuit for effecting a loop back operation in response to a predetermined state of signals received from two directions of the duplex loop lines, and a return indication circuit for adding, to a transmitted signal a return indication that a received signal was returned when the received signal could not be passed through the node.