A method for manufacturing a semiconductor elemental device including an SOI structure in which an SOI layer is laminated, includes the steps of setting transistor forming regions and a device isolation region to the SOI layer, forming a pad oxide film over the SOI layer and forming an oxidation-resistant film over the pad oxide film; forming a resist mask in a region corresponding to each of the transistor forming regions, and etching the oxidation-resistant film and the pad oxide film with the resist mask as a mask to expose the SOI layer of the device isolation region; removing the resist mask and oxidizing the exposed SOI layer by a LOCOS method using the oxidation-resistant film to form a field oxide film; and implanting amorphization ions in an edge portion formed in the SOI layer upon formation of the field oxide film to amorphize the edge portion.