To decide whether or not there is intra-device abnormality or oscillator abnormality by superposing a modulated signal of a low-frequency component on a main signal by an oscillator that each optical repeater is equipped with inside and sampling the low-frequency component by an opposite terminal station device as to a linear repeater using an optical direct amplifier, and to manage the whole system by the terminal station device at a time since the terminal station device is able to monitor the quality of an optical repeater through remote control.

This terminal station device superposes a transmission line abnormality monitoring acknowledgement signal on a main signal and the opposite terminal station device extracts a frequency component of the transmission line abnormality monitoring acknowledgement signal superposed on the main signal to decide whether or not there is transmission abnormality. Each repeater is equipped with a different oscillator, which is always driven in the absence of in-device abnormality to superpose the signal on the main signal. The opposite terminal station device extracts all frequency components to decide whether or not there is abnormality in the repeater and also decide which repeater has transmission abnormality from the frequency components extracted by the opposite terminal station device.