Each region, which should be left on a substrate after patterning, of a semiconductor film is grasped in accordance with a mask. Then, each region to be scanned with laser light is determined so that at least the region to be obtained through the patterning is crystallized, and a beam spot is made to hit the region to be scanned, thereby partially crystallizing the semiconductor film. Each portion with low output energy of the beam spot is shielded by a slit. In the present invention, the laser light is not scanned and irradiated onto the entire surface of the semiconductor film but is scanned such that at least each indispensable portion is crystallized to a minimum. With the construction described above, it becomes possible to save time taken to irradiate the laser light onto each portion to be removed through the patterning after the crystallization of the semiconductor film.