A semiconductor device includes a semiconductor region, a source region, a drain region, a source extension region a drain extension region, a first gate insulation film, a second gate insulation film, and a gate electrode. The source region, drain region, source extension region and drain extension region are formed in a surface portion of the semiconductor region. The first gate insulation film is formed on the semiconductor region between the source extension region and the drain extension region. The first gate insulation film is formed of a silicon oxide film or a silicon oxynitride film having a nitrogen concentration of 15 atomic % or less. The second gate insulation film is formed on the first gate insulation film and contains nitrogen at a concentration of between 20 atomic % and 57 atomic %. The gate electrode is formed on the second gate insulation film.