The present invention is to provide a laser irradiation technique for irradiating the irradiation surface with the laser beam having homogeneous intensity distribution using a cylindrical lens array without being affected by the intensity distribution of the original beam. A laser beam emitted from a laser oscillator is divided by two kinds of cylindrical lens arrays into a plurality of beams, which are two kinds of linear laser beams with their energy intensity distribution inverted each other, and the two kinds of linear laser beams are superposed in a minor-axis direction. This can form the linear laser beam having homogeneous intensity distribution on the irradiation surface.