PURPOSE: To suppress the inherent loss of an optical fiber to its minimum and to attain uniform light amplification by pumping an optical fiber span obtained by combining short optical fiber parts having added impurities and long optical fiber parts almost removing added impurities from two directions.

CONSTITUTION: An optical fiber span is formed so as to alternately include optical transmission medium part capable of compensating losses and optical transmission medium parts disabled at loss compensation by alternately arranging plural short optical fiber parts 12-2, 12-4, 12-6, 12-8 consisting of silica as a main component and containing added rare earth impurities and plural long optical fiber parts 12-1, 12-3, 12-5, 12-7, 12-9 corresponding to the fiber parts 12-2, 12-4, 12-6, 12-8, consisting of silica as a main component and almost removing added impurities. Amplifying optical fiber parts 12 to 14 are pumped by remote operation from either one of the end parts of the optical fiber span. Since pumping can be executed from two directions, i.e. from respective end parts of the optical fiber span, the uniformity of light amplification over the whole span can be improved. Consequently the optical transmission system including the light amplifier capable of obtaining almost uniform amplification can be obtained.