An object of this invention is to provide a semiconductor element manufacturing method in which, in forming a polycrystal semiconductor layer by applying ultraviolet rays to an amorphous semiconductor layer formed on a large substrate, an excimer laser employed in the conventional art is used in such a manner that the layer is made uniform in crystallinity, thereby to manufacture a polycrystal semiconductor layer high in quality. According to the present invention, in a semiconductor element manufacturing method comprising a step of applying an excimer laser beam providing a beam spot having a predetermined irradiation area to an amorphous semiconductor layer formed on an insulating substrate to crystallize the amorphous semiconductor layer to obtain a polycrystal semiconductor layer, the beam spot is moved over said amorphous semiconductor layer in a scanning mode while being shifted with a pitch of at most 1 mm, so that all the parts of the semiconductor layer are substantially equal to one another in the energy applied thereto.