To enable a cone angle to be set on the side of an object to be machined by a laser beam machine.

A laser beam emitted from a laser light source 2, after formed by a beam forming part 3, is transmitted or shielded in accordance with a prescribed pattern formed in a mask 4. The laser beam transmitted through the mask 4 is reflected by a dichroic mirror 5 and made incident on a disk 7. While the disk 7 is provided with plural transmitting parts of different areas, the transmitting part with a prescribed area is selected by a controller 1 in accordance with a desired cone angle, and inserted in the optical path of the laser beam by a motor 11. The laser beam transmitted through such part of the disk 7 is converged by an irradiation lens system 8 on the prescribed area of an object 9 to be machined; as a result, the object 9 is machined with the desired cone angle.