PURPOSE: To calculate the distance of a faulty point of an optical fiber transmission line including a repeater by connecting a transmitter and a receiver via a timing controller.

CONSTITUTION: A transmitter 8 is connected to a transmission section 1 of a terminal station 5, a reference bit is sent for each reciprocation time of a transmission line comprising optical fiber lines 6a, 6b and a repeater 7, a receiver 10 is arranged to a reception section 2 of the terminal station 5, a transmission signal sectioned by a reference bit is received and stored. Then the receiver 10 and the transmitter 8 are connected via a timing controller 9, when a fault takes place in the transmission line, the delay time from the reception of the reference bit of the transmission signal stored in the receiver 10 of both terminal stations 5 till the occurrence of the fault is compared respectively. Thus, the distance from both the terminal stations 5 till the faulty point is calculated.