A laser treatment apparatus is provided which is capable of irradiating a laser beam to the position where a TFT is to be formed over the entire surface of a large substrate to achieve the crystallization, thereby forming a crystalline semiconductor film having a large grain diameter with high throughput. A laser treatment apparatus includes a laser oscillation device, a lens for converging a laser beam, such as a collimator lens or a cylindrical lens, a fixed mirror for altering an optical path for a laser beam, a first movable mirror for radially scanning a laser beam in a two-dimensional direction, and an fθ lens for keeping a scanning speed constant in the case of laser beam scanning. These structural components are collectively regarded as one optical system. A laser treatment apparatus shown in FIG. 1 has a structure in which five such optical systems are placed. The number of optical systems is not limited; any number of optical systems is allowed as long as a means for supplying a plurality of laser beams is provided.