PURPOSE: To realize a high integration by reducing the area occupied in a separation region between elements for an integrated circuit substrate having SOI structure.

CONSTITUTION: A semiconductor substrate 1 is formed to a construction having one surface etched and arranged with a recess portion 2 and projected portion 3 alternately, a flat insulation layer 4 covering these recess portions and projected portions is formed, and the semiconductor substrate 1 is connected to a support substrate 7 through the insulation layer 4. The rear face of the semiconductor substrate 1 selectively etched so that regions corresponding to said recess portion 2 and projected portion 3 will have predetermined thickness. By doing this, a level difference is given to a semiconductor layer 1A on the insulation layer from an adjacent element region, by which elements are separated.