A galvanometer mirror rotates in one direction when the galvanometer mirror is used. A spot can be scanned on an irradiated surface at a more constant speed by rotating the galvanometer mirror and by using the inertia. Moreover, it is preferable to make the galvanometer mirror heavy because the inertia becomes higher so that the spot is scanned at a more constant speed. In addition, in a polygon mirror of this invention, mirrors are arranged so as not to contact each other because a change time of the scanning position between the mirrors is provided. By moving the irradiated object with timing together when the laser light is not irradiated, the laser process can be performed efficiently.