PURPOSE: To transmit the monitor state of a regenerative repeater to an end office, by providing a means into a regenerative repeater to detect a pulse train that is modulated by a specific frequency for monitor which is allotted to own.

CONSTITUTION: An end office 3 is connected to another end office (not shown in the diagram) via ascending and descending optical fibers 1 and 2 and numbers of regenerative repeater 4. When monitoring the repeater 4, the offices 3 modulates the monitor frequency signals supplied from an oscillator 51 by means of a pulse train sent from a pulse generator 52 in a signal transmitter 5 and then feeds the signals to the fiber 1. The signals regenerated by the regenerative repeater 4 are applied to a monitor device 43, and a detector 301 applies an output to a comparator 302 if the monitor frequecy is alloted to the comparator 302 itself. The comparator 302 controls gates 117, 118 and 123 in response to the result of comparison with a reference voltage generator 303. The signal of an oscillator 121 which has oscillations with the control voltage to a photoelectric converter 101 is modulated to a pulse and then fed to the signal receiver of the office 3 via a coupler 209 of a dvice 42 at the descending side. Receiving this signal, a frequency meter 63 and an error factor measuring device 7 indicate the state of the repeater 4.