To provide a laser irradiation apparatus and a laser irradiation method in which a region formed with microcrystals in a region irradiated with laser beams is decreased by disposing a slit in an optical system using a deflector, and laser processing can be favorably conducted to a semiconductor film. Further to provide a semiconductor manufacturing apparatus using the above-described laser irradiation apparatus and the laser irradiation method. In the optical system, an f-θ lens having an image space telecentric characteristic or a slit the shape of which is changed in accordance with the incidence angle of a laser beam, is used. The slit is disposed between the f-θ lens and an irradiation surface, and an image at a slit opening portion is projected onto the irradiation surface by a projection lens. By the above-described structure, laser irradiation can be uniformly conducted to a whole region scanned with laser beams.