In order to realize continuous gain control while realizing polarization independent amplifying characteristic, an optical amplifying device is configured to include: a Mach-Zehnder interferometer including an input side coupler, an output side coupler and a pair of arms, that connects the input side coupler to the output side coupler; a first and a second optical amplifier, to amplify signal light, which are disposed on the pair of arms respectively; and a light source for control light to output control light, in which the first and the second optical amplifier are configured to have polarization independent characteristic and the light source for control light is configured to have a predetermined polarization dependent gain difference or a predetermined polarization dependent loss difference.