Provided is a method of processing a workpiece, being capable of preventing refraction of a laser beam, caused by unevenness on the surface to be irradiated, and forming an appropriate modified layerinside the workpiece. The method of processing a workpiece includes the steps: a resin layer forming step of forming a resin layer through which a laser beam is transmitted on the back surface of theworkpiece; a step of sealing joint of a cover member: enabling a back surface of the cover member to be in sealing joint with the resin layer formed on the back surface of the workpiece, wherein the cover member enables the laser beam to penetrate through and is provided with a front surface that is more flat than the back surface of the workpiece; and a laser beam irradiating step of irradiatingthe laser beam to the workpiece from the exposed front surface side of the cover member to the workpiece via the resin layer to form a modified layer in the workpiece, wherein refraction, of the laserbeam, caused by the unevenness of the back surface is suppressed by the resin layer and the cover member.