PURPOSE: To facilitate maintenance and control of a transmission network by selecting different carrier frequencies by converters for each tip part of a transmission medium and giving the centralized monitor to the fault monitor information on each branch of the transmission line network.

CONSTITUTION: A transmission cable is divided into branches 29∼33, and converters 24∼28 are set to the tip parts of branches 29∼33 respectively. A center station 21 containing a head end 22 and a monitor device 23 is connected to an end of the transmission cable. Thus a monitor system is formed to a transmission line network. The frequency bands of this system are divided into ascending and decending bands 1 and 2. Then carrier frequencies f1∼fn and channels g1∼ gn are converted into frequencies F1∼Fn and channels G1∼Gn respectively by a head end 21. These frequencies F1∼Fn and channels G1∼Gn are compared with frequencies f1∼fn and channels g1∼gn respectively by converters 24∼28. Then coincident frequencies and channels are selected for response. The device 23 of the station 21 gives the centralized monitor to the fault monitor information of branches 29∼33 respectively.