An object to be processed 1 is irradiated with a laser light L modulated by a reflection type spatial light modulator 203 such that aberration of the laser light L converged inside the object 1 becomes a predetermined aberration or less. Therefore, aberration of the laser light L generated at a position on which a converging point P of the laser light L is located is made as small as possible, to enhance the energy density of the laser light L at that position, which makes it possible to form a modified region 7 with a high function as a starting point for cutting. In addition, because the reflection type spatial light modulator 203 is used, it is possible to improve the utilization efficiency of the laser light L as compared with a transmissive type spatial light modulator. Such improvement of the utilization efficiency of the laser light L is particularly important in the case in which the modified region 7 serving as a starting point for cutting is formed in the plate-shaped object 1.