a thickness of about 20 100 nm and then subjecting the silicon oxide film to rie as a result side walls of the gate insulating film 3 including a high dielectric constant material are covered with the disposable side walls 41 a therefore even when oxidation or a heat treatment or building up of an insulating film in a oxidizing gas containing atmosphere is carried out in the subsequent step s it is possible to prevent contamination of the semiconductor substrate 1 with a metal from the interface of the gate insulating film 3 or a lowering in relative permittivity of the gate insulating film 3 due to formation of a very thin silicon oxide film onto the interface of the semiconductor substrate 1 next as shown in fig5 b an impurity is introduced into an exposed surface layer of the semiconductor substrate 1 by ion implantation conducted by use of the disposable side walls 41 a and the like as a mask then an activating heat treatment for the thus introduced impurity is conducted to form source drain regions 17 in a surface layer of the semiconductor substrate 1 incidentally the ion implantation here is conducted by using a resist pattern as a mask to introduce an n type.