An object to be processed 1 comprising a substrate 4 and a plurality of functional devices 15 formed on a front face 3 of the substrate 4 is irradiated with laser light L while locating a converging point P within the substrate 4, so as to form at least one row of a divided modified region 72, at least one row of a quality modified region 71 positioned between the divided modified region 72 and the front face 3 of the substrate 4, and at least one row of an HC modified region 73 positioned between the divided modified region 72 and a rear face 21 of the substrate 4 for one line to cut 5. Here, in a direction along the line to cut, a forming density of the divided modified region 72 is made lower than that of the quality modified region 71 and that of the HC modified region 73.