In forming modification layers by radiating laser light to an inside of a processing target object from a modifying device periodically while rotating the processing target object held by a holder relative to the modifying device by a rotating mechanism and by moving the modifying device relative to the holder in a diametrical direction by a moving mechanism, a boundary position of the laser light in the diametrical direction is calculated, the boundary position being a position where a circumferential distance between the modification layers becomes a required threshold value, and a diametrical distance between the modification layers is reduced in a moving direction of the modifying device from the boundary position and/or a frequency of the laser light is reduced.