A modified region is accurately formed at a desirable position with respect to a laser light irradiation surface of an object to be processed. When an average difference γ has a value exceeding a predetermined threshold during trace recording, a particle segment Z including a line segment S where the average difference γ exceeds the predetermined threshold is defined. This determines that a particle exists on a line to cut 5 and randomly reflects measuring laser light, whereby a segment where the presence of the particle affects a control signal in a line segment to cut is detected as the particle segment Z. Correcting the control signal in the particle segment Z inhibits a converging lens from moving more than necessary because of an error included in the signal value under the influence of the presence of the particle, thus allowing the converging point of the processing laser light to accurately follow a front face 3.