An optical transmission system includes an optical transmission apparatus and an optical receiving apparatus. The optical transmission apparatus includes a transmitting unit that transmits signal light to the optical receiving apparatus through an optical transmission path, and a determining unit that determines continuity of the optical path using return light that returns from the optical receiving apparatus to the optical transmission apparatus through the optical transmission path, out of the signal light. The optical transmission apparatus includes a modulating unit that modulates a part of the signal light that is transmitted from the optical transmission apparatus through the optical transmission path to generate the return light, and an output unit that outputs the return light to the optical transmission apparatus through the optical transmission path.