A semiconductor device according to one embodiment includes: a semiconductor substrate having first and second regions; a first transistor comprising a first gate insulating film and a first gate electrode thereon in the first region on the semiconductor substrate, the first gate insulating film comprising a first interface layer containing nitrogen atoms and a first high dielectric constant layer thereon; a second transistor comprising a second gate insulating film and a second gate electrode thereon in the second region on the semiconductor substrate, the second gate insulating film comprising a second interface layer and a second high dielectric constant layer thereon, the second interface layer containing nitrogen atoms at an average concentration lower than that of the first interface layer or not containing nitrogen atoms, and the second transistor having a threshold voltage different from that of the first transistor; and an element isolation region on the semiconductor substrate, the element isolation region containing oxygen atoms and isolating the first transistor from the second transistor.