PURPOSE: To prevent a loop from being constituted between repeaters, by taking a signal of exclusive OR from a regenerating circuit signal and an opposed regenerating circuit signal which is looped back, and driving an optical shutter by its signal.

CONSTITUTION: A regenerating circuit 1 of an up-transmission line is connected to a shutter driving circuit 3, a delaying circuit and a gate circuit 10, and in the same way, a regenerating circuit 2 of a down-transmission line is connected to a shutter driving circuit 4, a delaying circuit 8 and a gate circuit 9. In such a state, in case of monitoring the down from the up, if the down is a non-signal, a transmission system goes to the down-transmission line through the regenerating circuit, a shutter 5 and the regenerating circuit 2. In such a case, by a signal of the gate circuit 9, the shutter 5 is opened. Subsequently, when a release signal is not sent out but a looping back signal is outputted, a loop of the regenerating circuit 2, a shutter 6, the regenerating circuit 1, the shutter 5 and the regenerating circuit 2 is formed, but by the signal of the gate circuit 9, the shutter 5 is closed, and the transmission system goes to the up-transmission line. Accordingly, it can be prevented to constitute a loop between repeaters.