A laser beam from an excimer laser is split into three beams along a first plane by a pair of prisms and the three beams are caused to intersect each other at the position of the object side focal point of a first cylindrical lens and be incident upon the first cylindrical lens. The three beams are respectively focused independently from each other by the first cylindrical lens. The above focused three beams are then focused along a second plane perpendicular to the first plane by a second cylindrical lens; then, the three beams are caused to superimpose each other on a mask and at the same time are brought to a defocus along the first plane and into focus again along the second plane by an anamorphic optical system containing a third cylindrical lens and a lens having a rotation symmetry, thereby a line-like illumination area extended in the first direction is formed on the mask. The linear illumination area and an area containing a pattern of a series of openings arranged in the first direction or a rectangular pattern extended in the first direction are caused to coincide on the mask to efficiently illuminate the pattern. An image of the illuminated pattern is projected onto a workpiece for an orifice plate by a projection lens system to process the workpiece in accordance with the pattern.