A semiconductor device includes an element isolation film formed on a semiconductor substrate surface of one conductivity type, a gate electrode having one pair of end portions located on a boundary between an element isolation film and an element forming region, a source region and a drain region of a reverse conductivity type arranged to sandwich a region immediately below a gate electrode, andan impurity diffusion region of the one conductivity type formed in the element forming region. The source region is separated from a region on a boundary side between the element isolation film and the element forming region in the region immediately below the gate electrode in the element forming region. In the impurity diffusion region, a portion adjacent to the region on the boundary side is arranged between the source region and the element isolation film, and is in contact with the source region and the region on the boundary side. The impurity diffusion region is not arranged between the drain region and the element isolation film.