A height position detector for detecting the height position of an upper surface of a work held on a chuck table, including: an annular spot forming part by which a spot of a laser beam oscillated by a laser beam oscillator is formed into an annular shape; a beam splitter by which the laser beam with the spot formed into the annular shape is guided into a first path; a light condenser by which the laser beam guided into the first path is condensed so as to irradiate the work held on the chuck table therewith; a conical mirror disposed in a second path into which the laser beam reflected by the work is split by the beam splitter, the conical mirror converting the reflected light with the annular spot shape into a linear spot shape; a position detector for detecting the position of the reflected light converted into the linear spot shape by the conical mirror; and a controller by which the height position of the upper surface of the work held on the chuck table is determined based on the position of the reflected light detected by the position detector.
