PURPOSE: To prevent a loop island caused at loopback.

CONSTITUTION: A control circuit consists of level detection circuits 32, 34 receiving a control signal 105 from an incoming transmission line and receiving a control signal 108 from an outgoing transmission line respectively and detecting the level, a switch 33 selecting a control signal whose level is detected, a reception circuit 38 decoding an on/off signal of loopback from the selected control signal, a NAND gate 35 and an AND gate 36 blocking an on-signal of loopback when an output signal 112 of the level detection circuit 34 is in existence an a signal latch S/R flip-flop 37. When other loopback circuit is formed with the control signal 108 from the outgoing transmission line, no loopback control signal 106 is outputted.