An optical device for optical communication includes a first main electrode disposed between a first splitter and a second splitter on a first arm. A first auxiliary electrode is disposed between the second splitter and a third splitter on the first arm. A second main electrode and a second auxiliary electrode are disposed between a third splitter and a fourth splitter on a second arm. The second main electrode is provided on the second arm at the first port side, and the second auxiliary electrode is provided on the second arm at the second port side. By such disposition of the first and second auxiliary electrodes, input signal light applied through a third port or a fourth port acts on the first main electrode prior to the first and second auxiliary electrodes. Therefore, the input signal light will not be affected by the first and second auxiliary electrodes.