A mounting table has a first mounting surface for mounting jigs sequentially one by one and a second mounting surface for mounting a ring member. The jigs respectively have facing portions facing an upper surface of the ring member, and positions of the facing portions in a radial direction of the ring member are different. An acquisition unit acquires gap information indicating a gap dimension between the second mounting surface and the facing portion of the mounted jig. A measurement unit measures a lifted distance of the ring member from the second mounting surface when the upper surface is in contact with the facing portion by lifting the ring member while the mounted jig is mounted. A thickness calculation unit calculates thicknesses of the ring member at different locations in the radial direction of the ring member based on the gap dimension and the measured lifted distance.