PURPOSE: To make the circuit configuration of a repeater station simple and low-priced and to monitor a fault at a switchboard by transmitting a fault display signal pattern to the switchboard by an optical fiber and monitoring the fault at the switchboard.

CONSTITUTION: The bipolar violation of a bipolar signal transmitted from a terminal station 11 is detected at a repeater station 29, the fault display signal pattern is inserted to the bipolar signal or a unipolar signal when detecting the bipolar violation, and this pattern is converted to an optical signal and transmitted to a switchboard 21 by an optical fiber 20. Therefore, the bipolar violation is announced to the switchboard 21 and since the switchboard 21 recovers the fault by switching a line or the like, a line terminal equipment and a switch control circuit are unnecessitated at the repeater station 29. Thus, the circuit configuration of the repeater station 29 is made simple and lowpriced and the fault is monitored at the switchboard.