To use an SOI process using a buried insulating layer by an oxygen ion implantation operation when a transistor is manufactured and to prevent a crystal defect from being generated by a method wherein a single-crystal Si layer for a thin-thickness channel region is formed and an Si layer for a source-drain region whose thickness is thicker than that of the channel region is formed.

A film 15 such as an SiO2 film which is capable of preventing ions from being implanted is generated on an Si substrate 11. The film 15 which is capable of preventing the ions from being implanted is patterned so as to be left only in a region which is to be used as the channel part of a transistor. Then, oxygen ions are implanted, and a buried SiO2 oxide layer 12, a damaged Si layer 13 and a single-crystal Si layer 14 are formed. Then, the film 15 is removed, the crystallinity of the damaged Si layer 13 is restored, a single-crystal Si layer 14' is formed, and an isolated single-crystal Si layer 16 is formed on the buried SiO2 layer 12. At this time, the single-crystal Si layer is formed in such a way that the thickness of the Si layer is thin in a part which is to be used as the channel region of a MOS semiconductor element and that the Si layer is thick in a part which is to be used as its source-drain region.