A semiconductor Mach-Zehnder optical modulator (200, 400, 500), comprising: an input waveguide; an optical demultiplexer for demultiplexing light inputted from the input waveguide; first and second arm waveguides (204, 205) for wave-guiding the light demultiplexed by the optical demultiplexer; an optical multiplexer for multiplexing the light wave-guided through the first and second arm waveguides (204, 205), respectively; and an output waveguide for outputting the light demultiplexed by the optical demultiplexer, the semiconductor Mach-Zehnder optical modulator (200, 400, 500) comprising: a first signal electrode formed in parallel with the first arm waveguide; a second signal electrode formed in parallel with the second arm waveguide; a plurality of first phase modulation electrodes that are branched from the first signal electrode and that are provided on the first arm waveguide in a discrete manner along the first signal electrode; a plurality of second phase modulation electrodes that are branched from the second signal electrode and that are provided on the second arm waveguide in a discrete manner along the second signal electrode; a first ground electrode parallelly provided along the first signal electrode; a second ground electrode parallelly provided along the second signal electrode; and a plurality of connection wirings to connect the first ground electrode and the second ground electrode among a plurality of points, wherein: a differential signal is inputted to or outputted from the first signal electrode and the second signal electrode, and the plurality of connection wirings adjacent to one another are provided with an interval 1/4 times or less smaller than the wavelength of a signal propagated through the first and second signal electrodes (207, 208), characterized in that the first and second arm waveguides (204, 205) have a waveguide structure in which a semiconductor substrate has thereon a first conductive semiconductor clad layer (303 1, 303 2), a non-doped semiconductor core layer (304 1, 304 2), and a second conductive semiconductor clad layer (305 1, 305 2) formed by being layered consecutively, when a distance between the first signal electrode and the second signal electrode is expressed as L1 and each of a distance between the first signal electrode and the first ground electrode adjacent to the first signal electrode and a distance between the second signal electrode and the second ground electrode adjacent to second signal electrode is expressed as L2, a relationship of "2L1 ≦ L2" is satisfied. a first signal electrode formed in parallel with the first arm waveguide; a second signal electrode formed in parallel with the second arm waveguide; a plurality of first phase modulation electrodes that are branched from the first signal electrode and that are provided on the first arm waveguide in a discrete manner along the first signal electrode; a plurality of second phase modulation electrodes that are branched from the second signal electrode and that are provided on the second arm waveguide in a discrete manner along the second signal electrode; a first ground electrode parallelly provided along the first signal electrode; a second ground electrode parallelly provided along the second signal electrode; and a plurality of connection wirings to connect the first ground electrode and the second ground electrode among a plurality of points, a differential signal is inputted to or outputted from the first signal electrode and the second signal electrode, and the plurality of connection wirings adjacent to one another are provided with an interval 1/4 times or less smaller than the wavelength of a signal propagated through the first and second signal electrodes (207, 208), characterized in that the first and second arm waveguides (204, 205) have a waveguide structure in which a semiconductor substrate has thereon a first conductive semiconductor clad layer (303 1, 303 2), a non-doped semiconductor core layer (304 1, 304 2), and a second conductive semiconductor clad layer (305 1, 305 2) formed by being layered consecutively, when a distance between the first signal electrode and the second signal electrode is expressed as L1 and each of a distance between the first signal electrode and the first ground electrode adjacent to the first signal electrode and a distance between the second signal electrode and the second ground electrode adjacent to second signal electrode is expressed as L2, a relationship of "2L1 ≦ L2" is satisfied.