PURPOSE: To eliminate the wrong transmission of signals due to a loop-back signal and to increase the signal transmitting distance, by transmitting signals to a repeater station at the center side from a remote repeater station only for a period needed for discrimination of signals.

CONSTITUTION: A repeater station 5 is connected to a center unit 1 via a signal line 2 connected to a terminal unit 10, and a remote repeating station 6 is connected to a signal line 3 for extension connected to a terminal unit 11. Then both stations 5 and 6 are connected via optical fibers 4a and 4b, and microcomputers μC50 and C60 containing ports K∼N and k∼n are provided to both stations 5 and 6 respectively. Thus the output timing is controlled for signals ViN on both signal lines 2 and 3. At the same time, the transmission times of signals sent from fibers 4a and 4b are also controlled when necessary. Then the signals are transmitted to the station 5 of the center side from the station 6 just in a period needed for discrimination of signals. This prevents the wrong transmission of signals due to a loop-back signal and increases the signal transmitting distance.