A nitride semiconductor device includes a substrate, a nitride semiconductor laminate, and an ohmic electrode of TiAl-based material. The nitride semiconductor laminate has a first nitride semiconductor layer on the substrate, and a second nitride semiconductor layer forming a heterointerface with the first nitride semiconductor layer. The nitride semiconductor device has an oxygen concentration profile in a depth direction of the device across between the ohmic electrode and the nitride semiconductor laminate. The profile has a first oxygen concentration peak near an interface between the ohmic electrode and the nitride semiconductor laminate in a region, of the nitride semiconductor laminate, that is on a substrate side of the interface, and a second oxygen concentration peak having an oxygen concentration of 3×1017 cm−3-1.2×1018 cm−3 in a position deeper than that of the first oxygen concentration peak.