A mach-zehnder optical modulator includes a substrate having electrooptical effect, wherein: the substrate has an inputting waveguide portion, two intermediate waveguide portions, an outputting waveguide portion and a sub optical waveguide portion, the inputting waveguide portion being branched and coupled to the two intermediate waveguide portions, the two intermediate waveguide portions being combined and coupled to the outputting waveguide portion, the sub optical waveguide being provided on at least one side of the outputting optical waveguide portion and having a given distance from the outputting optical waveguide portion, the sub optical waveguide portion guiding an off-light emitted from a combining portion of the two intermediate waveguide portions; and a width of a starting point of the sub optical waveguide portion is larger than the outputting waveguide portion.