Arranged on both sides of a thin plate are optical displacement gauges that irradiate measurement lights onto surfaces of the thin plate and receive the measurement lights reflected by the surface so as to measure displacements of the surfaces of the thin plate. Variation of thickness of the thin plate is obtained on the basis of the displacements of the surfaces of the thin plate measured by each of the optical displacement gauges. Each of the optical displacement gauges detects the displacement of a respective surface of the thin plate with high accuracy by irradiating the measurement light to the thin plate two times.
