PURPOSE: To immediately change the diameter of a laser beam, and to shorten a working time by setting the diameter dimension of a restriction which coincides with a beam axis M according to a command.

CONSTITUTION: A program for setting a beam diameter is read out from a working program memory 2 by a central processing unit 1, and a signal is given to a stop setting part 4 to set the dimension of a restriction 5. The working signal of the central processing unit 1 is received by a mirror drive part 3, and laser marking working is performed to an object to be worked by a laser beam with a beam diameter of a prescribed dimension. For setting the dimension of the restriction 5, restrictions which consist of the number from 2 to 20, with different diameters, are changed over. Or, an iris stop whose number of stop sheets is composed by either number from 3 to 30, is adopted. The device is equipped with two kinds of large and small laser beam diameters or over. Since a line can be drawn with a slender diameter by a beam setting command and a surface can be plotted by making immediately the slender diameter a large diameter, a graphic form on which line and surface exist, can be worked in a short time.