A field-effect transistor including N−-extension regions, an N+-drain region, an N+-source region and a gate electrode at a surface of a silicon substrate. A sidewall insulating film on one of the side surfaces of the gate electrode partially covers the surface of the N−-extension region, and a sidewall insulating film on the other side surface entirely covers the N−-extension region. Further, a silicon oxide film covers the surface of N−-extension region not covered by the sidewall insulating film. Thereby, resistances of the gate electrode, source region, and drain region can be easily reduced in a transistor having extension regions, which are asymmetrical with respect to the gate electrode.