An optical transmission system and an optical communication device

An optical transmission unit is comprised of an optical power control unit (14) to adjust the optical power, an optical detection unit to detect the power of the optical signal from a transmission medium (5), an optical control-information generating unit (17) to generate information relating to the size of the detected optical power monitor value, an optical control-information introduction unit to input the generated information into the transmission medium as optical control information, an optical control-information input detection unit to detect optical control information from the transmission medium (5), and a control unit (20) to regulate the detected information so the optical power from the optical power adjustment device is within a specified value.