PURPOSE: To shorten the recovery time of a system after fault occurrence as well as the time of detecting a fault spot, by employing the conception that faults are bracketed into electric-system faults and photo-system faults.

CONSTITUTION: In light transmitters 2' and 3', switching methods 25 and 35 are provided which send back electric signals from one transmission line to the other one. If a fault occurs, the switching methods are sequentially operated and transmitted signals are compared with sent-back signals to judge whether an electric- system fault has occurred. As a result, when it has occurred, electric system equipments 21 and 24 or 31 and 34 are checked and when the electric-system fault is found not to have occurred, photo-system equipments 22, 23, 32, 33, 5, and 6 are inspected.