PURPOSE: To serve two different communication protective systems by one device and to protect communication accompanying the change of a communication network form by providing a setting circuit for selecting a transmission line switching system by transmission line switching control information or signal quality monitoring.

CONSTITUTION: When the network form for protecting a line is set from an outside, a communication protective system setting circuit 111 closes a short-circuit switching device 106, connects an output signal switching circuit 110 to a side A and simultaneously stops the operations of path quality monitoring circuits 107 and 108 and a path quality comparing circuit 109. Corresponding to the contents of information extracted in line termination circuits 101 and 102, a line switching control information processing circuit 103 performs control for closing a switch 104 and opening the switch 105 when the signals of a transmission line 1 are valid and closing the switch 105 and opening the switch 104 when the signals of the transmission line 2 are valid. The switch 104 switches connection between the output signals of the termination circuit 101 and circuit output corresponding to the control signals of the processing circuit 103. Also, the switch 105 switches the connection between the output signals of the termination circuit 102 and the circuit output corresponding to the control signals of the processing circuit 103 and thus, the line is protected.