The main sensor 15 measures the distance Lm to the surface of the pad 2a, and the sub-sensor 16 measures the distance Ls to the surface of the reference block 12. What is actually taken as the measured value is the value of (Lm+Ls). The reference block 12 is used in order to give a reference position for measuring the surface position of the pad 2a. Accordingly, correct measurements can be performed even if the position of the movable element 9 should fluctuate, for example, as a result of deformation of the guiding and holding plate 7 or guide 8. When the motor 11 is caused to rotate, the ball screw 10 rotates, so that the movable element 9 moves leftward and rightward, and the distance to the pad 2a is measured. From the measured data of this distance, the circular-conical vertical angle, groove depth, thickness, and the like of the pad 2a are determined.