An objective of the present invention is to provide a laser crystallizing method capable of suppressing a thermal damage on a substrate as well as enhancing a substrate processing efficiency, and a laser irradiation apparatus using the laser crystallizing method. Laser lights oscillated from plural laser oscillating apparatuses are synthesized into one laser light and in a scanning direction of the laser light thus obtained, areas having an energy density lower than a predetermined level are cut with a slit. With the above construction, an average value of laser light energy densities can be increased in the scanning direction. Therefore, laser light irradiation time per area can be suppressed and in addition, a heat quantity applied to an object to be processed can be increased in total. Accordingly, a crystallinity of a semiconductor film can be increased while preventing the substrate from being excessively heated.