PURPOSE: To obtain a non-defect superthin film SOI substrate by forming an active region to become the SOI substrate simultaneously with an impurity infected region.

CONSTITUTION: A photoresist 2 is used as a mask to implant phosphorus ions 3 into a substrate 11. After the resist 2 is removed, an Si2N4 film 5 is formed by LP-CVD. Next, a photoresist 6 is used as a mask to etch the Si3N4 film 5 and the top 4a of an implanted region 4 at both ends by RIE to leave a part of the implanted region, which serves as an implanted region 4b. At this time, a planned part 300 for an SOI substrate is formed. Next, only the implanted region 4b is selectively etched to form a large gap 7. Then, thermal oxidation is conducted to fill the gap 7 with the oxide film. At this time, the top and the bottom of the SOI layer 300 are filled completely with oxide films 8 to form a superthin film SOI part 30 uniformly.