An optical amplifier includes a light source that outputs light having an intensity corresponding to a supplied driving power; a modulator that modulates the light output from the light source in response to an input modulation signal; a Raman amplifier that performs Raman amplification on the light modulated by the modulator using a highly-nonlinear medium; and a driver that supplies the driving power to drive the light source and inputs the modulation signal to the modulator. The driver starts inputting the modulation signal to the modulation unit before the intensity of the light propagating through the Raman amplifier exceeds an intensity value at which stimulated Brillouin scattering occurs in the Raman amplifier.