A nitride semiconductor device includes a multilayer semiconductor structure made of a group III nitride semiconductor and having a light-emitting facet, and a first coating film formed to cover the light-emitting facet of the multilayer semiconductor structure. The first coating film is a crystalline film made of a nitride containing aluminum. The crystalline film is composed of a group of single domains, and the single domain is comprised of a group of grains whose crystal orientation planes have a same inclination angle and a same rotation angle. A length of a boundary between the domains per unit area is 7 μm−1 or less.