PURPOSE: To provide an SOI substrate manufacturing method wherein element regions are formed uniform in thickness.

CONSTITUTION: Element regions S1 and S2 are formed on a first Si substrate 1, an SiO2 insulating layer 3 is deposited thereon, a second Si substrate 2 is pasted thereon, and then the first Si substrate 1 is polished to make the element regions S1 and S2 exposed for the formation of an SOI substrate, wherein etching is so carried out as to make the surface of the SiO2 insulating layer 3 flush with the exposed surface of the thinnest element region S1 and the element region S2 protrude from the surface of the SiO2 insulating layer 3. A piled layer 5 formed of material of the same polishing rate with the element regions S1 and S2 is formed on the surface and then polished again to make the element regions S1 and S2 equal in thickness.