PURPOSE: To realize a flat gain characteristic in a saturated area of an optical amplifier without lowering a level of an input light signal.

CONSTITUTION: When an exciting light is supplied from an exciting light source 6, an optical amplifier medium 3 amplifies an input light signal to output it. As it operates in a saturated area, a level of the input light signal becomes greater, and a gain becomes smaller. On the other hand, an optical amplifier medium 10 is constructed of the same medium as the optical amplifier medium 3, and as the exciting light is intercepted in an OBPF 4, it has an attenuation characteristic. As a damped ratio of the optical amplifier medium 10 is complementary to the optical amplifier medium 3, and the greater the level of the input light signal becomes, the smaller the damped ratio becomes, a gain characteristic integrated with the optical amplifier mediums 3, 10 is made flat even in the saturated area.