An optical amplifier comprising: an optical amplifying medium through which a wavelength division multiplex, WDM, light is arranged to travel, the WDM light including signal lights wavelength division multiplexed together; pump light sources (6-1, 6-2, 6-3) for generating at least three pump lights of different wavelengths; a first optical multiplexer for multiplexing the at least three pump lights together; a second optical multiplexer for multiplexing the WDM light with the multiplexed pump lights, the multiplexed pump lights thereby causing the WDM light to be amplified via Raman amplification in the optical amplifying medium as the WDM light travels through the optical amplifying medium; an input detector (11-1, 11-2, 11-3, 10) arranged to divide the WDM light before being amplified by the optical amplifying medium into wavelength bands and to detect the power in each of the wavelength bands; an output detector (7-1, 7-2, 7-3, 5) arranged to divide the amplified WDM light into wavelength bands and to detect the power in each of the wavelength bands; and a pump light controller arranged to control each power of the at least three pump lights based on the power detected by the input detector and the power detected by the output detector, by: calculating, for each of the wavelength bands, a difference between a gain calculated using each power of the input detector and the output detector for the wavelength band and a target gain being the average gain of a total wavelength band; determining, for each of the pump light sources (6-1, 6-2, 6-3), a control amount of the power level of the pump light source using the calculated difference; and changing the power of each of the at least three pump lights by the control amount. an optical amplifying medium through which a wavelength division multiplex, WDM, light is arranged to travel, the WDM light including signal lights wavelength division multiplexed together; pump light sources (6-1, 6-2, 6-3) for generating at least three pump lights of different wavelengths; a first optical multiplexer for multiplexing the at least three pump lights together; a second optical multiplexer for multiplexing the WDM light with the multiplexed pump lights, the multiplexed pump lights thereby causing the WDM light to be amplified via Raman amplification in the optical amplifying medium as the WDM light travels through the optical amplifying medium; an input detector (11-1, 11-2, 11-3, 10) arranged to divide the WDM light before being amplified by the optical amplifying medium into wavelength bands and to detect the power in each of the wavelength bands; an output detector (7-1, 7-2, 7-3, 5) arranged to divide the amplified WDM light into wavelength bands and to detect the power in each of the wavelength bands; and a pump light controller arranged to control each power of the at least three pump lights based on the power detected by the input detector and the power detected by the output detector, by: calculating, for each of the wavelength bands, a difference between a gain calculated using each power of the input detector and the output detector for the wavelength band and a target gain being the average gain of a total wavelength band; determining, for each of the pump light sources (6-1, 6-2, 6-3), a control amount of the power level of the pump light source using the calculated difference; and changing the power of each of the at least three pump lights by the control amount. calculating, for each of the wavelength bands, a difference between a gain calculated using each power of the input detector and the output detector for the wavelength band and a target gain being the average gain of a total wavelength band; determining, for each of the pump light sources (6-1, 6-2, 6-3), a control amount of the power level of the pump light source using the calculated difference; and changing the power of each of the at least three pump lights by the control amount.