In a thickness-measuring method during a grinding process, which suppresses wear of a probe coming into contact with a chuck table subjected to self-grind and accurately grinds a workpiece, measurement of wafer thickness is performed only at the commencement and termination of grinding, and a reference probe of a reference side height gauge is separated from the upper surface of the chuck table in the practical grinding time between the commencement and termination so as to interrupt the measurement of wafer thickness, thereby significantly reducing a time of the reference probe brought into contact with the upper surface of the chuck table.
