PURPOSE: To use a supervisory device in common and also simplify a supervisory signal, by using a nondense wave of a pulse signal as a fault supervisory signal between light terminal equipments, and also placing a detection circuit, a counting circuit and a light switching circuit.

CONSTITUTION: A supervisory signal for a nondense wave of a pulse signal is generated by the signal generator 33, and it is sent out to the main transmission line 18 from the light terminal equipment 11. And, this supervisory signal is detected by the detection circuit 36 which is provided on each light repeater 15, its output pulse is counted by the counting circuit 37, and when it has reached a prescribed value which has been decided at every light repeater 15, the light switching circuit 38 is closed. Subsequently, when the light switching circuit 38 is closed, the supervisory signal is returned to the main transmission line 19, and it is compared with a supervisory signal which has been sent out by the detector 34 of the light terminal equipment 11.