PURPOSE: To provide a laser machining method and its device which are capable of machining a part irradiated with a laser beam into a smooth finish.

CONSTITUTION: The wave length of a beam generated by a carbon dioxide gas laser device 121 is controlled into a single wavelength by selecting with a diffraction grating 112; and this laser beam with the single wavelength is converged while an assist gas is made to flow on the outer circumference of the laser beam along its transmitting direction. Then, on the optical path of the laser beam, a shutter 132 is arranged which is provided with a mirror function; and the output voltage of the drive source 138 is controlled so as to make the light power constant by monitoring the reflected light with an optical power meter 137.