A Raman amplifier for an optical transmission system. A light source unit generates pumping light to amplify wavelength-multiplexed signal light having a plurality of wavelengths including a wavelength band of the signal light. A transmission path propagates the signal light and the pumping light and amplifies the signal light. A pumping light supplying unit supplies the pumping light to the transmission path. A filter narrows a spread of a spectrum of the pumping light within the wavelength band of the signal light and outputs it to the pumping light supplying unit. The present invention increases transmission capacity while suppressing degradation of the transmission characteristic due to crosstalk.