An optical amplifier includes a first amplification medium to receive light obtained by combining signal light input into an input port and the excitation light generated by a light source; a second amplification medium disposed between the first amplification medium and an output port; a loss medium to receive the signal light separated from light output from the first amplification medium; a variable optical attenuator that is disposed on a path that bypasses the loss medium, and to receive the excitation light separated from the light output from the first amplification medium; a first photodetector to detect power of light separated from the signal light transmitted from the second amplification medium; and a controller to control the amount of attenuation for the variable optical attenuator or output power of the light source so that signal light power per wavelength of the signal light becomes closer to a target value.