An object of the present invention is to provide a method and a device for constantly setting the energy distribution of a laser beam on an irradiating face, and uniformly irradiating the laser beam to the entire irradiating face. Further, another object of the present invention is to provide a manufacturing method of a semiconductor device including this laser irradiating method in a process. Therefore, the present invention is characterized in that the shapes of plural laser beams on the irradiating face are formed by an optical system in an elliptical shape or a rectangular shape, and the plural laser beams are irradiated while the irradiating face is moved in a first direction, and the plural laser beams are irradiated while the irradiating face is moved in a second direction and is moved in a direction reverse to the first direction. The plural laser beams may be irradiated while the irradiating face is moved in the first direction, and the plural laser beams may be irradiated while the irradiating face is moved in the direction reverse to the first direction, and the irradiating face may be also moved in the second direction.