an optical amplifier includes : a rare - earth doped fiber configured to amplify signal light to thereby produce a amplified signal light ; a gain control circuit configured to control an optical gain of the rare - earth doped fiber ; a photodetector configured to detect intensities of different wavelength of light obtained from the amplified signal light ; and an abnormality detection circuit configured to detect an abnormality of the signal light in accordance with a ratio or a difference between the intensities of the different wavelength .