To provide a semiconductor device including a low-loss optical waveguide. The optical waveguide included in the semiconductor device has a core layer covered with first and second clad layers having respectively different refractive indices. A portion of the core layer is covered at a first ratio, that is, a ratio of the first clad layer to the second clad layer and at the same time, a second ratio, that is, a ratio of the second clad layer to the first clad layer. At this time, the first ratio and the second ratio are each a finite value more than 0.