A semiconductor device according to the present invention comprises a semiconductor substrate, a gate insulating film which is composed of a material whose main component is a tetravalent metal oxide, a mixture of a tetravalent metal oxide and SiO.sub.2, or a mixture of a tetravalent metal oxide and SiON and which containing B when it is in an nMOS structure on the semiconductor substrate or containing at least one of P and As when it is in a pMOS structure on the semiconductor substrate, and a gate electrode made of a metal having a work function of 4 eV to 5.5 eV.