Abstract: Provided are a laser apparatus of continuous oscillation that is capable of enhancing the efficiency of substrate processing, a laser irradiation method, and a manufacturing method for a semiconductor device using the laser apparatus. A portion of a semiconductor film that should be left on a substrate after patterning is grasped in accordance with a mask. Then, a portion to be scanned with a laser light is determined so that it is possible to crystallize at least the portion to be obtained through the patterning. Also, a beam spot is made to strike the portion to be scanned. As a result, the semiconductor film is partially crystallized. That is, with the present invention, the laser light is not scanned and irradiated onto the entire surface of a semiconductor film but is scanned so that at least an indispensable portion is crystallized.