In a semiconductor device including a plurality of element regions and an element isolation region based on STI (shallow trench isolation) which electrically isolates the element regions from each other, each of the element regions includes; a channel region; source/drain regions formed to sandwich the channel region in a horizontal direction; a gate insulation film which is formed on the channel region and in which an angle of a bird's beak is 1 degree or smaller, the bird's beak being formed from a side of the element isolation region on a surface opposite a surface facing the channel region in a horizontal direction substantially perpendicular to the direction in which the source/drain region sandwich the channel region; and a gate electrode layer formed on the gate insulation film.