To measure the length of an cylindrical dielectric resonator during the grinding operation in a condition where the effect of the oscillation caused by grinding is eliminated.

After the working position in the vertical direction of a piston unit 2 is set by a unit 3 to vertically drive a piston, the compressed air is introduced into a cylinder 22, and a dielectric resonator 7 fitted to a chuck unit 1 is pressed against a grinding member 43 arranged on a rotating grinding table 41 to grind the end face. A displacement measurement unit 5 emits the pulse laser beam L onto an end face of the piston through a laser beam L through a laser beam emission/reception part 51, obtains the change in the distance to the end face of the piston based on the time difference between the reception time of the reflected beam and the emission timing of the pulse laser beam L, and measure the length with the change in the distance as the grinding quantity of the dielectric resonator 7 during the grinding work.