Affords a manufacturing method enabling nitride-based semiconductor devices containing epitaxial films excelling in flatness and crystallinity to be easily produced, and makes available nitride-based semiconductor devices manufactured by the method. Method of manufacturing nitride semiconductor devices that are formed onto a semiconductor substrate being a compound containing a Group IIIA element for forming compounds with nitrogen, and nitrogen, including steps of heating the semiconductor substrate ( 1 ) to a film-deposition temperature, supplying to the substrate a film-deposition gas containing a source gas for the Group IIIA element and a nitrogen source gas, and epitaxially growing onto the semiconductor substrate a thin film ( 2 ) of a compound containing the Group IIIA element and nitrogen, and being furnished with a step, in advance of the epitaxial growth step, of heating the semiconductor substrate to a pretreating temperature less than the film-deposition temperature, to clean the surface of the semiconductor substrate.