A method for fabricating a semiconductor device includes: forming a first film on a nitride semiconductor layer so as to contact the nitride semiconductor layer and have a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, the first film being made of silicon nitride having a composition ratio of silicon to nitrogen larger than 0.75, silicon oxide having a composition ratio of silicon to oxygen larger than 0.5, or aluminum; and forming a source electrode, a gate electrode and a drain electrode on the nitride semiconductor layer.