A cylindrical lens (4) diverges a laser beam (L1) in the Y-axis direction (i.e., within the YZ plane) but neither diverges nor converges it in the X-axis direction (i.e., within the ZX plane). An objective lens (5) converges the laser beam (L1) emitted from the cylindrical lens (4) into a point P1 in the Y-axis direction and into a point P2 in the X-axis direction. As a consequence, the cross section of the laser beam (L1) becomes elongated forms extending in the X- and Y-axis directions at the points P1, P2, respectively. Therefore, when the points P1, P2 are located on the outside and inside of the work (S), respectively, an elongated working area extending in the Y-axis direction can be formed in a portion where the point P2 is positioned within the work (S).