Information of oscillation frequency of a laser beam source in a predetermined range including usable oscillation frequency that takes into consideration an output quality of the laser beam source, such as information on a relationship between oscillation frequency and output quality, is stored in a memory. Then, when the laser beam source is controlled, based on the information stored in the memory, a particular frequency range where the output quality of the laser beam source deteriorates or becomes good is specified, and a decision is made whether or not oscillation frequency f, which is going be used, is within the particular frequency range. Then, according to the results, the oscillation frequency is set, avoiding a frequency range where the output quality deteriorates or within a frequency range where the output quality becomes good according to the result. Thus, a laser beam source whose output quality is always good can be used.