PURPOSE: To obtain a high-reliability optical transmission unit by utilizing the solid wire two-way communication path of plural optical transmission fibers to return information indicating presence and absence of receiving signals to the transmission side and selecting a normal transmission path to insert signals of the faulty transmission system when the transmission path is faulty.

CONSTITUTION: In video modulator circuits 1 and 2, modulation signals of, for example, three video signals are mixed into one signal. A space of one-channel CH components is provided in arrangement of the modulation frequency, and circuits 1 and 2 are placed in positions corresponding to space channels of each other. Outputs of circuits 1 and 2 are connected to light emission circuits 5 and 6 respectively through switching circuit 4 and are transmitted to the receiving side through photo coupling branch equipments 7 and 8 and fibers 9 and 10. In case that one of fibers is faulty, it is inspected by detection circuits 15 and 16 whether a carrier wave exists or not, and this information is sent to switching circuit 22 and is changed to optical signals by light emission circuit 17 or 18, and these signals are sent to detection signal light receiving circuit 19 or 20 through photo coupling branch equipment 11 or 12 or fibers. Outputs of circuits 19 and 20 select a normal transmission system through switching circuit 21, and modulation signals of the faulty system are inserted into the space CH to transmit signals.