A dielectric insulator separated substrate comprises a plurality of monocrystalline semiconductor island regions in which circuit elements are to be formed and a support region for supporting the island regions while a dielectric film formed on the supporting region electrically separates the island regions from each other. The supporting region comprises crystalline semiconductor layers and at least one oxygen diffusion preventive film laminated alternately.The extreme outer polycrystalline semiconductor layer of the supporting region is polished to such a thickness as to prevent the substrate from being curved greatly by the wedge action due to the oxygen diffusion. Since the extreme outer polycrystalline semiconductor layer thus polished has a flat surface, the handling of the substrate is easy. The substrate devoid of any curveness deformation assures a highly accurate formation of the circuit elements in the island regions.