An optical
semiconductor device including: a substrate having a principal surface; a first and a second optical waveguides disposed on the principal surface of the substrate, the first and second optical waveguides extending in a first direction, the second optical
waveguide being arranged adjacent to the first optical
waveguide in a second direction intersecting with the first direction; a first and a second
signal electrodes disposed on the first and second optical waveguides; a
resistor disposed on the principal surface, the
resistor being arranged between the first optical
waveguide and the second optical waveguide, the
resistor being electrically connected to the first
signal electrode and the second
signal electrode; a resin layer disposed on the principal surface, top surfaces of the first and second signal electrodes, and the resistor; and a
capacitor disposed on the resin layer, the
capacitor being electrically connected to the resistor through an opening of the resin layer.