To provide an optical amplifier wherein a change in wavelength dependency of a gain with the intensity of input light is suppressed, and hence the wavelength dependency of a gain is left unchanged over a wide range of input light intensity.

There are provided a first optical amplifier excited by first excitation light for amplifying input signal light, a population inversion generating section for generating population inversion for an element or an ion performing an optical amplification operation by supplying second excitation light to the first optical amplification section, and a first control section for controlling the intensity of the first excitation light with the foregoing gain and a loss by estimating the loss of the second excitation light and the gain of the signal light by the first optical amplification section.