An N-MOSFET is formed on an SOI substrate consisting of a semiconductor substrate, an insulating layer and an n−-active layer. A p-well layer, an n-RESURF layer, and an n-diffusion layer are formed in the surface of the n−-active layer between a source electrode and a drain electrode by means of impurity diffusion. The diffusion regions of the p-well layer and the n-RESURF layer overlap with each other. An end of the n-RESURF layer reaches a position below a gate electrode. The diffusion regions of the p-well layer and the n-diffusion layer do not overlap with each other, so that the n-RESURF layer has a region in direct contact with the n−-active layer between the p-well layer and the n-diffusion layer.