Abstract: A monitoring control system for a digital transmission line including above one repeater provided between two end stations uses a monitoring control signal of an alternating code with the polarity thereof inverted at a frequency or out-of-serice of a digital transmission line. The alternating code is transmitted, with the polarity thereof inverted at a frequency determined by monitoring control information, from an information transmission side between the repeater and one of the end stations for the transfer of control information. On an information reception side therebetween an exclusive OR of a received signal and an alternating code obtained through frequency division of a clock signal extracted from the received signal is obtained. The frequency component is reproduced from the exclusive OR. And the monitoring control information transmitted from the information transmission side is identified in a step of generation of the reproduced frequency component.