To provide a method for stably forming a device separation layer in a thin semiconductor layer and prevent formation of a bird's beak. The method for forming the device on a semiconductor substrate having a support substrate and the semiconductor layer above the support substrate is provided with: a step for preparing the semiconductor substrate where a transistor forming region and an element separation region are set; a step for forming a pad oxide film on the semiconductor layer of the semiconductor substrate; a step for forming an oxidation-resistant mask layer on the pad oxide film; a step for forming a resist mask covering the transistor forming region on the oxidation-resistant mask layer; a first etching step for etching the oxidation-resistant mask layer with the resist mask as amask and exposing the pad oxide film of the element isolation region; and a step for removing the resist mask, oxidizing the semiconductor layer below the pad oxide film exposed with the exposed oxidation-resistant mask layer as the mask by a LOCOS method and forming the element separation layer.