A nitride semiconductor device includes a substrate, a stacked semiconductor structure formed over the substrate and including a electron channel layer of an undoped nitride semiconductor and an electron supplying layer of an n-type nitride semiconductor formed epitaxially over the electron channel layer, the n-type nitride semiconductor having an electron affinity smaller than an electron affinity of said undoped nitride semiconductor and a two-dimensional electron gas being formed in the electron channel layer along an interface to the electron supply layer, a gate electrode formed over the stacked semiconductor structure in correspondence to a channel region, and source and drain electrodes formed over the stacked semiconductor structure in ohmic contact therewith respectively at a first side and a second side of the gate electrode, the stacked semiconductor structure including, between the substrate and the electron channel layer, an n-type conductive layer and a barrier layer containing Al formed consecutively and epitaxially.