A repeater station 10 detects whether or not an abnormality exists in each of signal light and monitor light having arrived from a terminal station 1 or a repeater station 20, and controls, according to the result of detection, operations of optical amplifiers 11 and 12 and the monitor light. In the case where both of the signal light and monitor light arriving from the terminal station 1 via the optical transmission line for the downstream direction are abnormal, the optical amplifier 11 for the downstream direction stops its amplifying operation by itself, whereas a monitor control apparatus 13 stops the amplifying operation of the other optical amplifier for the upstream direction and also stops transmitting the monitor light to the terminal station 1 in the upstream direction.