In the case of a lens array type homogenizer optical system, the incident angle and intensity of a laser beam 1 entering a large-sized lens (long-axis condenser lens 22) of a long-axis condensing optical system, which is provided on the rear side, are changed for every shot by performing laser irradiation while long-axis lens arrays 20a and 20b are reciprocated in a direction corresponding to a long axial direction of a linear beam (X-direction). Therefore, vertical stripes are significantly reduced. Further, the incident angle and intensity of a laser beam 1 entering a large-sized lens (projection lens 30) of a short-axis condensing optical system, which is provided on the rear side, are changed for every shot by performing laser irradiation while short-axis lens arrays 26a and 26b are reciprocated in a direction corresponding to a short axial direction of a linear beam (Y-direction). Therefore, horizontal stripes are significantly reduced.