A semiconductor device includes: a first gate insulating film formed on a first nMOS transistor region in a semiconductor substrate; a second gate insulating film formed on a first pMOS transistor region in the substrate; a third gate insulating film formed on a second nMOS transistor region in the substrate; and a fourth gate insulating film formed on a second pMOS transistor region in the substrate. The first through fourth gate insulating films contain nitrogen. Each of the third and fourth gate insulating films has a thickness smaller than that of each of the first and second gate insulating films. The first gate insulating film has a nitrogen concentration peak at the interface between the first gate insulating film and the substrate. Each of the second, third and fourth gate insulating films has a nitrogen concentration peak only near an associated one of gate electrodes respectively formed thereon.