An optical device includes an optical coupler that performs branching of the input light and outputs a first-type branched light and a second-type branched light; a polarization converter that changes the direction of polarization of the second-type branched light output from the optical coupler; and a polarization synthesizer that outputs a polarization multiplexed light by synthesizing the first-type branched light, which is output from the optical coupler, and the second-type branched light, which has the direction of polarization changed by the polarization converter. The optical coupler has the wavelength characteristic that cancels out the wavelength characteristic of the polarization synthesizer regarding the second-type branched light included in the polarization multiplexed light.