An illumination optical system has superior image formation capabilities, and
can radiate a linear beam with excellent illumination uniformity and a narrow line
width having a large aspect ratio; the illumination optical system comprises an
afocal beam expander system which expands an beam illuminated from a laser light
source; a linear beam-forming lens system having at least refractive power in a
second direction which is substantially at a right angle to at least a first direction,
the linear beam-forming lens system converting the beam, illuminated from said
beam expander system, to a linear beam having its long side in said first direction;
a lens array section having a plurality of element lenses, arranged along said
first direction; and a condenser optical system which illuminates a processed face
by reconnecting images of said linear beam from each of said element lenses thereon.
