To obtain a laser beam machining method and its device capable of enhancing reproducibility of high quality machining.

As a laser beam machining method, in the case of forming an optical system provided with an aperture 3 in the optical path of a laser beam from a laser generator 4 to the lens, an intensity distribution is detected for the laser beam passing through the aperture 3 and, on the basis of the detected information, the laser beam diameter through the aperture 3 is adjusted for the machining.