It is an object of the present invention to provide a laser irradiation technique which can keep the stability of the laser oscillator high and which can perform laser process homogeneously by avoiding the adverse effect due to the return light reflected on an irradiation when, for example, crystallizing with a lens array, and to provide a crystallization method and a method for manufacturing a semiconductor device which use the technique. In the present invention, a laser beam emitted from a laser oscillator is divided into a plurality of beams through a lens array such as a cylindrical lens array, the divided beams pass through opening portions of a slit while being focused at the opening portions and condensed beam is irradiated to an irradiation surface. Thus, the light reflected on the irradiation object can be blocked by using the slit.