An optical SSB modulator capable of appropriately adjusting a phase difference of light in the MZ waveguides and automatically adjusting a voltage impressed to the bias adjustment electrodes is provided. The above-mentioned object is achieved by an optical SSB modulator (1) provided with a first sub Mach-Zehnder waveguide (MZA) (2), a second sub Mach-Zehnder waveguide (MZB) (3), a main Mach-Zehnder waveguide (MZC) (4), a first bias adjustment electrode (DCA electrode) (5), a second bias adjustment electrode (DCB electrode) (6), a first modulation electrode (RFA electrode) (7), a second modulation electrode (RFB electrode) (8) and a third bias adjustment electrode (DCC electrode) (9), wherein either one of or both of output portions (10, 11) of the MZA and the MZB have an X-branching form, and specifically having one of light paths of the X-branching connected to a photodetector.