A method of manufacturing a semiconductor device includes the steps of emitting a plurality of laser beams out of a plurality of lasers, synthesizing the plurality of laser beams into a laser light wherein centers of two adjacent laser beams in the laser light are distant from each other, and irradiating the laser light to a semiconductor film, wherein a distribution of an energy density of the laser light in a longitudinal direction in the laser light is within .+-.10% except for attenuation regions, thereby uniformly annealing the semiconductor film.