To prevent the damage of a substrate during positioning or transporting of the substrate.

Lifting parts 82a, 82b provided in a substrate holder 82 are elevated to cause a substrate G mounted on the substrate holder 82 to be strained, a light emitted from a projector 83 is reflected on the downside of the strained substrate G and incident on a photoreceiver 84, the photoreceiver 84 is a position detecting element capable of detecting the beam position of the incident light, and thus the thickness of the substrate G is measured, based on the detection result of the position of the incident beam variable according to the strain quantity of the substrate G.