To achieve the high-speed line machining in a shape having less energy losses and high edge-effect (in other words, a machined portion and a non-machined portion are clearly different from each other) in a laser beam machining apparatus.

The laser beam output from a laser head 1 is formed by an expander optical system 2 to generate parallel beam. At least a part of the formed parallel beam is blocked as necessary by a diaphragm part 3. The formed laser beam is emitted to an object 7 to be machined which is placed on an automatic conveying stage 6 via the diaphragm part 3, a laser beam optical lens barrel 4, and an object lens 5, in order to perform machining. The machining can be performed in a shape having less energy losses and high edge-effect by forming the laser beam by the expander optical system 2, and subjecting the formed laser beam to diaphragm processing.