PURPOSE: To eliminate the loop-back in a semi-duplex bidirectional optical communication system, by stopping the operation of a transmission system during the receiving of optical data, because either a transmission or the receiving system is inoperative in case of the semi-duplex communication.

CONSTITUTION: Electric cords 69, 71 connect respectively between a terminal controller 31 and an optical data set 78 and between a terminal device 34 and an optical data set 79. Further, the two optical data sets 78, 79 are connected by two optical cords 73, 74. A receiving circuit of an electric signal is made inoperative while the optical data sets 78, 79 receive an optical signal, so as not to loop a signal at input and output terminal subjected to wired-OR.