An optical system that controls laser beam spot profile for forming a high performance thin film by a laser heat treatment process is provided. In the optical system that irradiates a rectangular laser beam on a film formed on a substrate, intensity distribution forming, apparatus makes the intensity distribution uniform in the longitudinal direction while maintaining the properties of the laser beam 2 such as directivity in the direction of shorter side, making it possible to concentrate the light to a limit permitted by the nature of the laser beam and achieve the maximum intensity gradient on the film disposed on the substrate. Thus a steep temperature distribution can be generated on the film disposed on the substrate and, as a result, high performance thin film can be formed.