PURPOSE: To enable accurate measurement in a wide range, by supporting a movable member having a sensing pin mounted thereto by an air bearing.

CONSTITUTION: A pad 34 constituting an air bearing is provided inside a Z-frame 32. The pad 34 is formed into a disc shape and air is injected from the hole formed to the plate surface thereof to support the frame 32 so as to float the same from a guide 30. Therefore, the frame 32 freely slides to a Z-direction. An intermediate guide 38 is fixed to the under surface of the frame 32 and the frame body 40A of a Y-frame 40 is externally fitted to said guide 38. Pads 40 are provided to the upper, under, left and right surface of the frame body 40A and the frame 40 is supported by the guide 38 so as to be made freely movable to the Y-direction. Further, the square pole shaped part 46A at the upper end of an X-frame 46 is engaged with the guide hole in the frame body of the frame 40. Pads 48, 52, 50 are provided to the upper, under, left and right surfaces of the guide hole and the frame 46 is supported by the guide hole so as to be made freely movable to the X-direction. A sensing pin 54 is provided to the end part 46B of the frame 46 and parallelly moved to arbitrary X-, Y- and Z-directions to perform measurement.