To provide a continuous-oscillating laser apparatus capable of improving the efficiency of substrate treatment, a method of irradiating a laser beam, and a method of manufacturing a semiconductor device using the laser apparatus. Of the entire semiconductor film, a portion that needs to be left on the substrate after patterning is identified according to a mask. Then, a portion to be scanned by respective lasers are defined, so that a laser beam is irradiated twice in different scanning directions to a portion to be obtained at least through patterning and beam spots are impinged upon the scanned portion, thereby partially crystallizing the semiconductor film. In other words, in the invention, it is arranged in such a manner that a laser beam is not irradiated by scanning a laser beam across the entire semiconductor film but by scanning a laser beam twice at least to the absolutely necessary portion. According to the above arrangement, it is possible to save the time to irradiate a laser beam in waste to the semiconductor film at a portion to be removed through patterning, and the crystalline characteristics of the semiconductor film obtained after the patterning can be further enhanced.