An object of the present invention is to reduce the manufacturing cost of a semiconductor device. A semiconductor device includes a SOI substrate that has an optical waveguide including a semiconductor layer. The optical waveguide is covered with an interlayer insulating film. Wiring parts are formed on the interlayer insulating film. Moreover, a thin film part having a smaller thickness than the wiring parts is formed above the optical waveguide and is integrated with the wiring parts.