PURPOSE: To transfer the image of a laser beam just after taken out from a laser beam oscillator to a rectangular mask even when using a rod as a laser beam medium or even when using a slab whose ratio of a thickness and a width, differs from the aspect ratio of the mask.

CONSTITUTION: In a mask system laser beam marker 100, a rod is used on a solid state laser beam oscillator. Prisms 4a and 4b are inserted between a total reflection mirror and an output mirror, and a laser beam flared only in the lateral direction, is taken out from the output mirror. A convex lens 6a has the function of an image-formation optical system making the image of a laser beam in the position of the output mirror enlarge and project on the position of a mask. A laser beam irradiates so that a marking pattern in a rectangular mask, is successfully covered.