PURPOSE: To stably continue laser beam processing by constituting the titled mechanism in such a manner that a holding circuit for setting the lighting period of laser light sets the longer lighting period of the laser light when the change-over of the selection of a selecting circuit is executed by a prescribed number of times within the prescribed time.

CONSTITUTION: An adequate signal level is not attained even if either output signal is selected and therefore the selecting circuit 13 changes over the selection alternately when the laser light is irradiated on the boundary between a glass surface and chromium surface. The selection signal 20c thereof is inputted to the holding circuit 16. The holding circuit 16 lights up a semiconductor laser 1 usually in a relatively short period. On the other hand, the holding circuit operates to change-over the lighting period of the semiconductor laser 1 to the substantially longer period when the on and off of the selection signal 20c inputted from the selecting circuit 13 are repeated many times within the specified time. The focal position of the laser light is maintained as it is when the laser light is assumed to be focused to the glass surface. The adjustment of the focal position is executed again after the signal 20c of the circuit 13 is stabilized to the on or off.