The purpose of the present invention is to provide a technique of manufacturing a nitride semiconductor layer with which, when producing a semiconductor device by forming a nitride semiconductor layer on off-angle inclined substrate, it is possible to stably supply high-quality semiconductor devices by preventing occurrence of a macro step using a material that is not likely to occur lattice strains or crystal defects by mixing with GaN and does not require continuous addition; and provided is a nitride semiconductor device which comprises a nitride semiconductor layer formed on a substrate, wherein the substrate is inclined at an off angle, a rare earth element-added nitride layer to which a rare earth element is added is formed on the substrate as a primed layer, and a nitride semiconductor layer is formed on the rare earth element-added nitride layer.