An improvement is achieved in the reliability of a semiconductor device. Over an insulating layer, an optical waveguide and a p-type semiconductor portion are formed. Over the p-type semiconductor portion, a multi-layer body including an n-type semiconductor portion and a cap layer is formed. Over a first interlayer insulating film covering the optical waveguide, the p-type semiconductor portion, and the multi-layer body, a heater located over the optical waveguide is formed. In the first interlayer insulating film, first and second contact holes are formed. A first contact portion electrically coupled with the p-type semiconductor portion is formed continuously in the first contact hole and over the first interlayer insulating film. A second contact portion electrically coupled with the cap layer is formed continuously in the second contact hole and over the first interlayer insulating film. A wire formed over a second interlayer insulating film is electrically coupled with the heater and the first and second contact portions via plugs embedded in the second interlayer insulating film.