PURPOSE: To realize the regenerating system in which the regeneration of a monitoring signal is independent of the marking rate of a main signal pulse train in a regenerating system for the monitoring signal in a digital optical transmission line.

CONSTITUTION: The system is provided with a 1st peak detection section part 1 whose time constant is selected to be sufficiently larger than one bit length of a main signal pulse train M and sufficiently smaller than one period of a modulation signal, a band filter 2 whose center frequency is selected to be a modulation signal frequency, and a 2nd peak detection section 3 whose time constant is selected to be sufficiently larger than one period of the modulation signal. Then transmission line signals M, S are detected by the 1st peak detection section 1, the signals pass through the band pass filter 2 and detected by the 2nd peak detection section 3.