Laser light is converged at an object including a semiconductor substrate formed with a plurality of functional devices on a front surface, from a back surface of the semiconductor substrate, and while a distance between the front surface and a first converging point of the laser light is maintained at a first distance, whereby a first modified region is formed along the line. The laser light is converged at the object from the back surface, and while a distance between the front surface and a second converging point is maintained at a second distance, and while the second converging point is offset with respect to a position at which the first converging point is converged, whereby a second modified region is formed along the line. A predetermined portion including the back surface and at least the second modified region is removed.