PURPOSE: To enable a light amplifier system with a low noise and a high- saturation output to be achieved without any influence of multiple reflection at a high gain by using a light amplifier with larger noise and saturation output at a later stage of a light amplifier at a low-noise previous stage.

CONSTITUTION: An optical fiber amplifier 1 at a previous stage is constituted with a wave synthesizer 13 for synthesizing signal light to be amplified and excitating light from an excitation light source 11 and allowing it to enter an amplification medium 12. Then, an optical fiber amplifier 2 of a later stage is constituted by a wave synthesizer 23 for allowing excitating light from an excitation light source 21 to enter from a signal input side of an amplification medium 22. Also, light signal for monitoring is branched by a branch equipment 31 and an AGC part 30 is provided to allow output light intensity to be maintained to be constant, thus obtaining a low noise and high-saturation output, preventing influence of multiple reflection which becomes prominent at high-gain operation effectively, and obtaining a higher S/N ration even at a lower input than before for increasing transmission distance drastically.