Abstract: A semiconductor device including a substrate having a low substrate surface formed in the substrate with a first gentle slope from the substrate surface; a single crystalline layer formed on the low substrate surface nearly level with the substrate surface and having a gentle slope facing the first gentle slope; an optical semiconductor element is constructed using the single crystalline layer. An electronic semiconductor element is constructed using the substrate surface. A wiring layer connects electrodes of the optical semiconductor element and the electronic semiconductor element through the first and the second gentle slopes.