A semiconductor device includes pMISFET and nMIS formed on the semiconductor substrate. The pMISFET includes, on the semiconductor substrate, first source/drain regions, a first gate dielectric formed therebetween, first lower and upper metal layers stacked on the first gate dielectric, a first upper metal layer containing at least one metallic element belonging to groups IIA and IIIA. The nMISFET includes, on the semiconductor substrate, second source/drain regions, second gate dielectric formed therebetween, a second lower and upper metal layers stacked on the second gate dielectric and the second upper metal layer substantially having the same composition as the first upper metal layer. The first lower metal layer is thicker than the second lower metal layer, and the atomic density of the metallic element contained in the first gate dielectric is lower than the atomic density of the metallic element contained in the second gate dielectric.