PURPOSE: To switch a line to a standby line, by causing the code pattern of monitor signals to be sent out to a time division multiplex device in its own station as the code pattern of fault information upon detecting fault information in its own station, disconnection of monitor signals from an opposite station, or code pattern of fault information from the opposite station.

CONSTITUTION: The time division multiplex device 5 of a slave station 12 connected with the line terminal of a branch line section 22a at which a fault occurs detects line switching signals from a monitoring board 4 after detecting fault information. The time division multiplex devices 2 and 8 of a host station 11 and another slave station 13 which are respectively connected to the line terminals of a base line section 21a and branch line section 23a, at which no fault occurs, respectively detect line switching signals from monitoring boards 1 and 7. In any case, by means of the series of operations, the time division multiplex devices 2, 5, and 8 can detect any one of the line switching signals or fault information DNR, REC, SEND, and UNR and switching from the currently used line 31 to a standby line 32 can be performed after monitoring operations are carried out for a fixed time.