Abstract: A wear rate measurement method includes thermally coupling a focus ring having a top surface and a bottom surface with a reference piece having a bottom surface facing a susceptor and a top surface facing the focus ring; measuring a first optical path length of a low-coherence light beam that travels forward and backward within the focus ring by irradiating the low-coherence light beam to the focus ring orthogonally to the top surface and the bottom surface thereof; measuring a second optical path length of a low-coherence light beam that travels forward and backward within the reference piece by irradiating the low-coherence light beam to the reference piece orthogonally to the top surface and the bottom surface thereof; and calculating a wear rate of the focus ring based on a ratio between the first optical path length and the second optical path length.