An illumination beam irradiation apparatus for use in pattern inspection systems is disclosed, which is less in deterioration of optical components and in attenuation of illumination light. The illumination apparatus includes a light source which yields a fundamental wave, a beam-shaper unit which performs beam-shaping of the fundamental wave so that this wave has a prespecified shape, and a pattern generator unit which operates, upon receipt of the beam-shaped fundamental wave, to convert this incoming wave into illumination light with a shorter wavelength to thereby generate illumination light of a prespecified shape. The illuminator also includes an image relay unit for guiding the illumination light that was generated by the pattern generator to fall onto a workpiece under inspection, such as a photomask or else.