PURPOSE: To generate a vivid marking by providing a pattern formed from a diffusive surface and a reflection film on the substate of a mask made of a material to admit passage of laser beam, and precluidng leaving a mark of laser irradiation around the marking pattern.

CONSTITUTION: A laser beam 6 is cast onto a mask 12, and the beam having passed the mask 12 is passed through an image forming lens 4 to be cast onto a work to be processed 5. At this time, image formation of the pattern 13 formed on the mask 12 is made by the lens 4 on the work 5. On the beam incident side of the substate 7 of the mask 12, the areas other than the pattern 13 are made a diffusive surface 8, and the lasr beam diffused by this surface 8 is reflected by a reflection film 9 so that laser beams from the areas other than the pattern 13 are shut off completely. Because the reflection film 9 is irradiated only with the laser beams which were diffused by the diffusive surface 8, the film 9 will never be damaged. On the beam emission side of the substate 7, the areas other than the pattern 13 are left in the form of reflection film 9.