A Raman amplifier which can suppress the increase of MPI and enhances the amplification efficiency of signal light, and an optical transmission system using such amplifier are provided. The Raman amplifier comprises an optical fiber 10 having a reflection point at least at a first point 11 or second point 12, and a forward pumping source 21 and backward pumping source 22. In the pumping sources 21 and 22, the conditions for providing pumping light, such as whether to provide forward and backward pumping light and the power distribution between forward and backward pumping light, are designed in accordance with the location of the reflection point and reflection factor thereof in the optical fiber.