An optical transmission system which permits transmission distance to be prolonged without using repeaters and yet ensures economical, high-quality optical transmission. A branch station performs non-repeated communication with an optical branching point and includes a light pumping section for causing pump light to enter an optical fiber through which a branched, receiving optical signal flows, to perform optical amplification by using the fiber as an amplification medium. An optical branching device includes an optical amplification section and an optical branching section. The optical amplification section redirects the pump light originated from the branch station and propagated through a line to the paired line through which an optical signal transmitted from the branch station flows, to excite an amplification medium inserted in the paired line and doped with active material for optical amplification and thereby amplify power of the optical signal transmitted from the branch station. The optical branching section branches the optical signal transmitted from the branch station and optical signals transmitted from other stations.