Provided are a semiconductor optical modulator and a semiconductor Mach-Zehnder optical modulator of high efficiency and high reliability.The semiconductor optical modulator 10 of the present invention includes a substrate 11; a first n-type cladding layer 12; a semiconductor optical modulation layer 13; a p-type cladding layer 14; a second n-type cladding layer 15; a passivation film 19; and an electric field-relaxing layer 16, wherein the first n-type cladding layer 12, the semiconductor optical modulation layer 13, the p-type cladding layer 14, and the second n-type cladding layer 15 are laminated on the substrate 11 in this order to form a waveguide structure, the passivation film 19 is arranged at the side surfaces of the waveguide structure, the electric field-relaxing layer 16 is interposed between the p-type cladding layer 14 and the second n-type cladding layer 15, and an impurity concentration of the electric field-relaxing layer 16 is lower than that of the p-type cladding layer 14 and that of the second n-type cladding layer 15.