A method for fabricating a semiconductor device includes forming a device isolation film on a substrate between first and second regions, forming first and second sealing films, such that an etch selectivity of the second sealing film is smaller than that of the first sealing film, patterning the first and second sealing films to expose the second region and a portion of the device isolation film, such that an undercut is defined under a lower surface of the second sealing film, forming a filling film filling the undercut, a thickness of the filling film being thicker on a side surface of the second sealing film than on an upper surface thereof, removing a portion of the filling film to form a filling spacer in the undercut, forming a high-k dielectric film and a metal film on the filling spacer, and patterning the high-k dielectric film and the metal film.