A wafer processing method including a modified layer forming step of applying a laser beam having a transmission wavelength to a substrate from the back side of the substrate along division lines. The modified layer forming step includes the steps of making the polarization plane of linearly polarized light of the laser beam parallel to the direction perpendicular to each division line, shifting the beam center of the laser beam from the optical axis of a focusing lens of a focusing unit for focusing the laser beam, in the direction perpendicular to each division line, and shifting the focal point of the laser beam by the focusing lens in the same direction as the direction where the beam center of the laser beam has been shifted.