PURPOSE: To easily and arbitrarily produce the optimum machining region of a laser oscillator in the laser beam machine to execute laser beam machining by condensing laser beam and irradiating work.

CONSTITUTION: The laser beam 10 emitted from a output mirror 22 of a laser oscillator 1 does not have the propagation characteristic, which causes the part where a beam diameter is made smaller in the course of propagation, but is diffused with a diffusing angle. On the way of optical path like this of laser beam 10, an optical part 3 having focus distance F is placed at the position of distance L from the output mirror 22. By this optical part 3, the laser beam 10 modified its optical path is turned to a laser beam 10a. Thus, by the optical part 3, a propagation characteristic of the laster beam 10 is changed, as a result, a position of the optimum machining region R is changed. That is, based on a focus distance F of the optical part 3 and a distance L from the output mirror 22, a position of the optimum machining region R is changed, by appropriately selecting the focus distance F and placing distance L, the optimum machining region R is arbitrarily produced.