The semiconductor device includes a semiconductor substrate, a gate insulating film formed in contact with an upper side of the semiconductor substrate, and a gate electrode formed on the upper side of the gate insulating film and made of metal nitride or metal nitride silicide. A buffer layer for preventing diffusion of nitrogen and silicon is interposed between the gate insulating film and the gate electrode. Preferably, the buffer layer has a thickness of 5 nm or less. In the case where gate electrode contains Ti elements, and the gate insulating film contains Hf elements, the buffer layer preferably contains a titanium film.