A hair-line processing, and more particularly a sheet metal finished by continuous hair-line on its plane and curved surface, and apparatus and method for finishing by continuous hair-line on the same are disclosed, wherein the sandpaper curvaceously advances to form hairlines relative to curved surface of the sheet metal, and operation of a pressure unit can vary the curvaceous degree of the sandpaper to form a continuous hairline relative a sheet metal including curved surfaces each having a different curvature.