A nitride semiconductor device 2 comprises a nitride semiconductor layer 10. A gate insulating film 16 is formed on the surface of the nitride semiconductor layer 10. The gate insulating film 16 includes a portion composed of an aluminum nitride film 15 and a portion composed of an insulating material 14 that contains at least one of oxygen or silicon. A region W2 of the nitride semiconductor layer 10 facing the aluminum nitride film 15 is included in a region W1 of the nitride semiconductor layer 10 facing a gate electrode 18. The nitride semiconductor device 2 may further comprise a nitride semiconductor lower layer 8. The nitride semiconductor layer 10 may be stacked on the surface of the nitride semiconductor lower layer 8. The nitride semiconductor layer 10 may have a larger band gap than that of the nitride semiconductor lower layer 8 and have a heterojunction formed there between.
