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 (MZ.sub.A) (2), a second sub Mach-Zehnder waveguide (MZ.sub.B) (3), a main Mach-Zehnder waveguide (MZ.sub.C) (4), a first bias adjustment electrode (DC.sub.A electrode) (5), a second bias adjustment electrode (DC.sub.B electrode) (6), a first modulation electrode (RF.sub.A electrode) (7), a second modulation electrode (RF.sub.B electrode) (8) and a third bias adjustment electrode (DC.sub.C electrode) (9), wherein either one of or both of output portions (10, 11) of the MZ.sub.A and the MZ.sub.B have an X-branching form, and specifically having one of light paths of the X-branching connected to a photodetector.