PURPOSE: To prevent generation of crystal defect and achieve sufficient pressure proof for a thin oxide film on the wafer surface by making construction of a layer with oxygen concentration not larger than a predetermined value, a layer with oxygen concentration gradually increasing with depth, and layer with oxygen concentration not less than the other particular value, from the mirror surface side.

CONSTITUTION: Construction is made of a layer 1 with oxygen concentration not larger than about 2×1016cm-3, a layer 2 with oxygen concentration gradually increasing with depth, and a layer 3 with oxygen concentration not less than about 9×1018cm-3, from the mirror surface side. Thus, large mechanical strength can be achieved, warpage due to heat treatment can be prevented, crystal defect is not produced even when a refined device is formed, and moreover, sufficient pressure proof can be assured for a thin oxide film on the wafer surface.