Setting in advance an absorptance of a light L (for example laser light) of a brittle material W to be processed, an absorptance of a plate-shaped sample S of the same material as the brittle material W to be processed is calculated using that set value of absorptance, a thickness of the tabular sample S when that sample S is irradiated by the light L having a same wavelength as that irradiated onto the brittle material, and based on this calculated value of absorptance, and the actual absorptance data obtained by the actual irradiation of the light L on the sample S, a light wavelength that is selected appropriate for the absorptance of the brittle material to be processed is selected, and processing of the brittle material W is carried out using light of the selected wavelength.