Herein disclosed is an optical modulator, comprising: a substrate 1 having an electro-optic effect; an optical waveguide 12 embedded in the substrate 1 to have a light wave guided therein; a traveling wave electrode 4 mounted on the substrate 1 to have a traveling wave applied thereon so that the light wave is modulated by the traveling wave with the electro-optic effect, the traveling wave electrode being constituted by a center electrode 4a and ground electrodes 4b, 4c; in which the optical waveguide has a plurality of interaction optical waveguides 12a, 12b to collectively form a Mach-Zehnder optical waveguide operative to modulate the light wave in a phase modulation manner under the condition that the traveling wave is applied to the traveling wave electrode, the interaction optical waveguides 12a, 12b collectively form a region where respective widths of said interaction optical waveguides are different from each other, and the center electrode 4a and the ground electrodes 4b, 4c are positioned such that interaction efficiencies between the high frequency electric signal and the light wave guided in the respective interaction optical waveguides 12a, 12b are substantially equal to each other.