Crystallization by irradiation of laser light forms a plurality of convex portions (ridges) on the surface of a crystalline semiconductor film that is obtained, which decreases a film quality. A method of laser irradiation comprises the steps of: overlapping an area which is irradiated with a first laser beam of a pulsed oscillation having a wavelength of equal to or shorter than that of visible light with an area which is irradiated with a second laser beam of a pulsed oscillation having a longer wavelength than that of the first laser beam; and irradiating a subject to be irradiated with the first laser beam and the second laser beam while synchronizing the pulsed oscillation of the first laser beam with that of the second laser beam, and moving the subject to be irradiated, and the first laser beam and the second laser beam relatively each other.