PURPOSE: To decrease defects in pictures, by forming a double-stage well struc ture by one-time well diffusion by using an epitaxial wafer, and preventing the exposure of an element forming region in a substrate during said formation.

CONSTITUTION: A p- type epitaxial layer 20 is provided on an n- type Si substrate. An SiO2 film 21 having a thickness of about several hundreds∼2,000 and a resist layer 22 are laminated. A marker 23 is opened in the resist 22, and the SiO2 layer 21 is etched so as to provide a hole. The resist 22 is separated and the surface is coated by resist 24 again. Position alignment is performed at the marker 23. Photoetching is performed, a hole is provided and B+11 is implanted. The resist 24 is removed. Resist 25 is newly applied. By utilizing the marker 23, position aligning is performed. Photoetching is performed and P+31 is implanted. The mask 25 is removed. Treatment is performed in N2 partially including O2 at 1,100∼1,250°C. Deep wells 26, isolation from the p- well and an (n) layer 27 are completed. Since the concentrations of the p- layer 20 and the n- layer 27 are about the same, the profile of the shallow p- well is not changed. In this method, the surface of an element forming region is not exposed, mixing of impurities and occurrence of crystal defects are decreased and defects in pictures are few.