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.