A semiconductor device and a method of manufacturing the same capable of reducing variations in the thickness of a semiconductor device are provided. The amount of oxygen implanted ions is less than the amount of implanted oxygen ions in the conventional epitaxial SIMOX wafers. Oxygen is ion-implanted into the surface layer of a silicon wafer from the surface of the wafer. Then, by heat treating the wafer, a thinning stop layer, which is an imperfect buried oxide film, is formed along the entire plane of the wafer. As a result, variation of the thickness of the semiconductor device formed in an active layer can be reduced, since the, the reliability of the accuracy of the end point of silicon wafer thinning is higher than that of a thinning using the conventional deep trench structure as an end point detector.