Over a semiconductor substrate, a silicon nitride film is formed so as to cover n-channel MISFETs. The silicon nitride film is a laminate film which may be made of first, second, and third silicon nitride films. The total film thickness of the first and second silicon nitride films is smaller than half a spacing between a first sidewall spacer and a second sidewall spacer. After being deposited, the first and second silicon nitride films are subjected to treatments to have increased tensile stresses. The total film thickness of the first, second, and third silicon nitride films is not less than half the spacing between the first and second sidewall spacers. The third silicon nitride film is not subjected to any tensile-stress-increasing treatment, or may be subjected to a lesser amount of such treatment.