To reduce a production cost of a semiconductor device and provide a semiconductor device having improved characteristics. A grating coupler has a plurality of projections separated from each other in an optical waveguide direction and a slab portion formed between any two of the projections adjacent to each other and formed integrally with them; a MOS optical modulator has a projection extending in the optical waveguide direction and slab portions formed on both sides of the projection, respectively, and formed integrally therewith. The projection of the grating coupler and the MOS optical modulator is formed of a first semiconductor layer, a second insulating layer, and a second semiconductor layer stacked successively on a first insulating layer, while the grating coupler and the MOS optical modulator each have a slab portion formed of the first semiconductor layer.