affords a manufacturing method enabling nitride - based semiconductor devices containing epitaxial films excelling in flatness and crystallinity to be easily produced . method of manufacturing nitride semiconductor devices that are formed onto a semiconductor substrate being a compound containing nitrogen , and a group iiia element for forming compounds with nitrogen , including steps of : heating the semiconductor substrate 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 of a compound containing nitrogen and the group iiia element ; 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 .