Disclosed herein is a semiconductor device chip manufacturing method including a chipping prevention layer forming step of forming a chipping prevention layer at each intersection of a plurality of crossing division lines formed on the front side of a wafer, a modified layer forming step of applying a laser beam having a transmission wavelength to the wafer to the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer, thereby forming a modified layer inside the wafer along each division line, and a dividing step of grinding the back side of the wafer after performing the modified layer forming step, thereby reducing the thickness of the wafer and also dividing the wafer into individual semiconductor device chips along each division line where the modified layer is formed as a break start point.