It is an object of the present invention to provide an optical element which forms a beam spot with homogeneous energy distribution on an irradiated surface, and a light irradiation apparatus. It is another object of the invention to provide a manufacturing method of a semiconductor device using a beam spot with homogeneous energy distribution. One feature of the invention is to homogenize energy distribution of a beam spot on an irradiated surface by forming the optical element provided with a polygonal entrance and exit using a plurality of reflectors as side walls and introducing a beam to the optical element. Further, a beam spot with a desired size or shape can be obtained by making the reflectors movable. Furthermore, by using a light irradiation apparatus using the optical element, failure in manufacturing a semiconductor device can be reduced.