A Mach-Zehnder optical modulator includes a join-and-branch portion; two light wave guides connected with the join-and-branch portion; an output light waveguide connected with the join-and-branch portion; arm electrodes respectively provided on the two light waveguides; and a ground electrode that is provided at an edge of at least one of the arm electrodes along the light waveguide, is spaced from the arm electrode and is grounded.