PURPOSE: To eliminate the need for special pre- and post-treatments at the time of executing laser beam processing by using a laser beam which is hardly absorbed.

CONSTITUTION: A computer causes an aq. copper sulfate soln. to be discharged to a point P so that the soln. is applied like a spot on this point upon recognition of the positioning of a soln. discharge point 5a at the point P which is the processing start point of a substrate 1. The computer, then, moves an X-Y-Z stage to align an optical axis 18 and the point P and instructs, at the point of this time, a laser oscillator to output the laser beam. The laser beam 23 cast to the point P is absorbed in the aq. copper sulfate soln. to raise the soln. to a high temp. and the substrate 1 in the lower part thereof is made into a molten state. The subsequent laser beam processing is well executed even if the aq. copper sulfate soln. is not applied. Namely, the cutting processing from the point P to the point Q is well executed without the application of the aq. copper sulfate soln. A cutting groove 1a is consequently formed on the substrate 1.