PURPOSE:To simplify the manufacturing process by a method wherein a mask layer of the same quality as that of a gate electrode is formed on a surface of the boundary zone of a drain region and a field insulating film to permit a source region and the drain region to be formed. CONSTITUTION:The field insulating film 9 is selectively formed on a semiconductor substrate having a (-) conductive type to effect marking an element forming region. Then, a gate insulating film 4 is forming on the element formed region, and then, the gate electrode 3 is formed on the gate insulating film 4. The mask layer 3' is in the zone where the gate insulating film 4 and the field insulating film 9 continue each other. Subsequently, a reverse conductive type impurity is introduced in the element forming region with the gate electrode 3 and the mask layer 3' as the masks.