Provided is a semiconductor device capable of having a single metal/dual high-k structure with a good shape and having flat band voltages suited for nMOS and pMOS, respectively. The semiconductor device according to the one embodiment of the present invention has a first conductivity type MOSFET and a second conductivity type MOSFET. The first and second conductivity type MOSFETs are each equipped with a first insulating film formed over a semiconductor substrate, a second insulating film formed over the first insulating film and made of an insulating material having a higher dielectric constant than the first insulating film, and a gate electrode formed over the second insulating film and having, as a lower layer of the gate electrode, a metal layer containing a material which diffuses into the second insulating film to control a work function thereof. The second conductivity type MOSFET is equipped further with a diffusion barrier film formed between the first insulating film and the second insulating film to prevent diffusion of a work-function controlling material into the interface of the first insulating film.