PURPOSE: To improve the transmission quality by providing one monitor means monitoring a line fault of a subscriber line per system, monitoring the presence of the line fault for each subscriber line periodically and separating completely and isolating the subscriber line from other circuits at the detection of a fault.

CONSTITUTION: A monitor controller 6 sends a start signal to a central controller 8 and a monitor 5 periodically, the central controller 8 receives the start signal to select sequentially one subscriber circuit 1 among all subscriber circuits 1 and to switch a test changeover circuit 11 to the position of the monitor 5. The monitor 5 monitors the presence of a line fault of a connected subscriber line 2 and the monitor controller 6 receives a monitor resulting signal representing fault detection from the monitor 5 and recognizes a line fault, then the central controller 8 controls a disconnection switch 12 of the selected subscriber circuit 1 thereby completely disconnecting and isolating the accommodated subscriber line 12 from other circuits. Thus, it is not required to prepare a monitor circuit for ground contact or the like to each subscriber circuit and the transmission characteristic is improved.