When a CW laser is irradiated to a semiconductor film while scanning relatively during a production process of a semiconductor device, elongated crystalline particles extending in the scanning direction are formed. The semiconductor film thus formed has characteristics substantially equal to a single crystal in the scanning direction. However, since the CW lasers is highly likely to induce interference, uniform laser irradiation is difficult to conduct. In this regard, interference of a laser beam can be decreased by setting the angle of incidence of the laser beam with respect to the surface of the semiconductor film is to be a desired angle other than 0.degree.. In general, the output of the CW laser is small, so that the laser beam has to be scanned reciprocally in order to irradiate a region of a large area. However, as the angle of incidence is not set at 0.degree., the effect of the laser beam irradiation differs between the outward trip and the return trip. To decrease this, the angle of incidence is made variable so that the condition of the laser beam irradiation can be equal between the outward trip and the return trip.