Providing a method and apparatus for efficiently irradiating a uniform laser light on an irradiation surface even when a laser light of high coherence or a large size substrate is used. The laser irradiation apparatus of the invention comprises a laser; means for dividing a laser light emitted from the laser into plural laser beams; means for synthesizing the laser beams on the irradiation surface or place in the vicinity thereof thereby forming a laser light having a periodical energy distribution; and means for moving the substrate relative to the laser light. Such a laser irradiation apparatus may be used to anneal the overall surface of a semiconductor film.