A first nitride semiconductor layer contains Ga. The first nitride semiconductor layer is, for example, a GaN layer, an AlGaN layer, or an AlInGaN layer. Then, an aluminum oxide layer has tetra-coordinated Al atoms each surrounded by four ◯ atoms and hexa-coordinated Al atoms each surrounded by six ◯ atoms as Al atoms in the interface region with respect to the first nitride semiconductor layer. The interface region is a region apart, for example, by 1.5 nm or less from the interface with respect to the first nitride semiconductor layer. Then, in the interface region, the tetra-coordinated Al atoms are present by 30 at % or more and less than 50 at % based on the total number of Al atoms.