PURPOSE: To attain stable line relieve at very high speed by providing a retrieval order designation device deciding the priority of standby lines to be retrieved depending on a position of a faulty active line in terms of radio frequency arrangement.

CONSTITUTION: An active line fault discrimination device 9 monitors active line fault detectors 6, 7 to recognize in which line a fault takes place and informs the result to a retrieval order designation device 11, which informs a predetermined retrieval order to a standby line retrieval device 10 by taking the frequency arrangement of the active line having the fault into account. The standby line retrieval device 10 activates a standby line fault detector 8 depending on the order of retrieval to retrieve the standby line able to be switched thereby giving a command to a switch controller 12. Then the switch controller 12 based on the command to select a sending end switch 3 and a receiving end switch 5 so as to apply line switching. Thus, the standby line is retrieved according to the decided retrieval order. Thus, the stable line relieve is attained at very high speed.