A Semiconductor device includes an insulating layer, an optical waveguide, a first dummy semiconductor film, a second semiconductor film and a third semiconductor film. The optical waveguide is formed on the insulating layer. The first dummy semiconductor film is formed on the insulating layer and is spaced apart from the optical waveguide. The first dummy semiconductor film is formed on the first semiconductor film. The second semiconductor film is integrally formed with the optical waveguide as a single member on the insulating layer. The third semiconductor film is formed on the second semiconductor film. A material of the first dummy semiconductor film is different from a material of the optical waveguide. In plan view, a distance between the optical waveguide and the first dummy semiconductor film in a first direction perpendicular to an extending direction of the optical waveguide is greater than a thickness of the insulating layer.