A first buried layer of one conductivity and a first well region of opposite conductivity are formed in a semiconductor layer using a first mask. A second mask is used to form a second buried layer and a second well region of the opposite conductivity and to introduce impurity of the one conductivity type into a surface of the second well to form a channel doped layer of the one conductivity. A high concentration buried layer of the opposite conductivity is formed by introducing opposite conductivity impurity into the second well region using a third mask. A gate insulating film is formed on the semiconductor layer by thermal oxidation. A source region and a drain region of the one conductivity type are formed on the surface of the second well region, on both sides of a gate electrode.