A substrate dividing method comprising the steps of: irradiating a semiconductor substrate, the semiconductor substrate having a front face on which a plurality of functional devices are formed, with laser light while positioning a light-converging point within the semiconductor substrate, so as to form a molten processed region (7, 13) due to multiphoton absorption only within the semiconductor substrate, the molten processed region (7,13) forming a starting point region for cutting along a line along which the semiconductor substrate should be cut, inside the semiconductor substrate by a predetermined distance from a laser light incident face of the substrate; placing a protective film over the front face of the semiconductor substrate after the said irradiating step, to hold the semiconductor substrate; grinding and then chemically etching a rear face of the semiconductor substrate after the step of placing the protective film over the front face of the semiconductor substrate to attain a predetermined thickness, whereupon the semiconductor substrate is divided into a plurality of chips by fractures generated from the molten processed region (7, 13) along the line along which the semiconductor substrate should be cut and reaching the front face and the rear face of the semiconductor substrate; attaching an expansion film to rear faces of the chips after the step of dividing the semiconductor substrate into the chips; removing the protective film from the front face of the semiconductor substrate; and expanding the expansion film after the step of attaching the expansion film, so as to separate the chips from each other. irradiating a semiconductor substrate, the semiconductor substrate having a front face on which a plurality of functional devices are formed, with laser light while positioning a light-converging point within the semiconductor substrate, so as to form a molten processed region (7, 13) due to multiphoton absorption only within the semiconductor substrate, the molten processed region (7,13) forming a starting point region for cutting along a line along which the semiconductor substrate should be cut, inside the semiconductor substrate by a predetermined distance from a laser light incident face of the substrate; placing a protective film over the front face of the semiconductor substrate after the said irradiating step, to hold the semiconductor substrate; grinding and then chemically etching a rear face of the semiconductor substrate after the step of placing the protective film over the front face of the semiconductor substrate to attain a predetermined thickness, whereupon the semiconductor substrate is divided into a plurality of chips by fractures generated from the molten processed region (7, 13) along the line along which the semiconductor substrate should be cut and reaching the front face and the rear face of the semiconductor substrate; attaching an expansion film to rear faces of the chips after the step of dividing the semiconductor substrate into the chips; removing the protective film from the front face of the semiconductor substrate; and expanding the expansion film after the step of attaching the expansion film, so as to separate the chips from each other.