When a laser bean temporally modulated in amplitude by a modulator is shaped into a long and narrow beam by a beam shaper, the scanning-direction size of the long and narrow beam shaped by the beam shaper is selected to be in a range of from 2 to 10 microns, preferably in a range of from 2 to 4 microns and the scanning speed of the beam is selected to be in a range of from 300 to 1000 mm/s, preferably in a range of from 500 to 1000 m/s. As a result, damage of the silicon thin film can be suppressed while energy utilizing efficiency of the laser beam can be improved. Accordingly, laterally grown crystals (belt-like crystals) improved in throughput can be obtained on a required region of a substrate scanned and irradiated with the laser beam.