While a converging lens 108 is relatively moved along a line 50 including a line to cut 5, measuring laser light L2 is converged with the lens 108 when the lens 108 is positioned on a processing region 30 having an outer shape between an object to be processed 1 and a frame 22, and a reflected light component of the laser light L2 reflected by a front face 3 of the object 1 is detected. While keeping a substantially constant distance between the front face 3 and the lens 108 such that a converging point P of processing laser light L1 is positioned at a constant distance from the front face 3 according to the detection, the laser light L1 is converged by the lens 108, so as to form a molten processed region 13 within the object 1.