An optical transmission system including: first, second, and third stations; a main transmission path that bi-directionally couples the first station with the second station; and an optical add/drop multiplexer (OADM) disposed on the main transmission path, the OADM branching light from the first and second station, combining light from the third station to the main transmission path; wherein the OADM includes an input determination unit and an optical path switching unit, the input determination unit detects the power of the lights, and determines whether each of the input lights is lost or not, and the optical path switching unit forms a loop-back route based on the determination.