An optical amplifier comprising: a pumping light source supplying a pumping light to an optical fiber as an amplification medium; an ASE light power detector detecting an ASE light power including an external ASE power flowing from an upstream side outside an amplification signal band; and a control unit setting a gain within the amplification signal band by using the ASE light power detected by the ASE light power detector outside the amplification signal band. The control unit controls the pumping light source by compensating for an influence of the external ASE power, obtained by measuring a relationship between the gain within the amplification signal band and the ASE light power outside the amplification signal band, to set initially the gain within the amplification signal band.