It is an object of the present invention to provide a nitride semiconductor device with low parasitic resistance by lowering barrier height to reduce contact resistance at an interface of semiconductor and metal. The nitride semiconductor device includes a GaN layer, a device isolation layer, an ohmic electrode, an n-type Al0.25Ga0.75N layer, a sapphire substrate, and a buffer layer. A main surface of the n-type Al0.25Ga0.75N layer is on (0 0 0 1) plane as a main surface, and concaves are arranged in a checkerboard pattern on the surface. The ohmic electrode contacts the sides of the concaves of the n-type Al0.25Ga0.75N layer, and the sides of the concaves are on non-polar surfaces such as (1 1 −2 0) plane or (1 −1 0 0) plane.