A Raman amplifier can easily reduce the gain variation in Raman amplification even when the power of an input signal varies. The Raman amplifier is provided with (a) an optical fiber that Amplifies an input lightwave to produce an output lightwave, (b) a pump-lightwave-supplying means that supplies pump lightwaves having a plurality of wavelengths to the optical fiber, and (c) a control unit that controls only the power of the pump lightwave having the shortest wavelength among the pump lightwaves so that the average value of the gain of the Raman amplification by the optical fiber in an intended wavelength range can be maintained constant.