A method for fabricating a Mach-Zehnder modulator includes: forming a resin body embedding a semiconductor mesa for an arm waveguide, the resin body having an opening on an upper face of the semiconductor mesa; forming an electrode on the semiconductor mesa and the resin body, the electrode being in contact with the upper face of the semiconductor mesa through the opening of the resin body; forming an inorganic insulating protective layer on the electrode and the resin body, the inorganic insulating protective layer having an arrangement of multiple first openings therethrough to the electrode; and forming a metal body on the inorganic insulating protective layer and the electrode, the metal body being in contact with the electrode through the multiple first openings of the inorganic insulating protective layer.