A semiconductor device includes: a semiconductor substrate including an active region and a gate structure on the active region. The gate structure includes a gate insulating film; a work function adjusting film on the first gate insulating film; a separation film on the work function adjusting film; and an oxygen capturing film on the separation film and configured to capture oxygen introduced from the outside of the first gate structure. The oxygen capturing film is spaced apart from a top surface of the first gate insulating film by about 70 Å to about 80 Å.
