Performance of a semiconductor device having a MIS transistor is improved. A semiconductor device includes: a pair of source/drain regions each formed by stacking a semiconductor layer on a main surface of a silicon substrate; a sidewall insulating film covering each sidewall of the source/drain regions; a gate electrode arranged so as to interpose a gate insulating film on the main surface of the silicon substrate at a position sandwiched by the sidewall insulating films in a plane; and extension regions formed to extend from a portion below and lateral to the gate electrode to a portion below and lateral to each of the source/drain regions, wherein a sidewall of the sidewall insulating film being adjacent to the gate insulating film and the gate electrode has an inclination of a forward tapered shape.