PURPOSE: To largely take measuring quantity and to enable accurate measurement, by supporting each frame by an air bearing and connecting the same in a chain form prior to assembling.

CONSTITUTION: A pad 34 constituting an air bearing is provided inside a Z-frame and supported so as to be floated from a guide 30. The upper end of 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 connected to the square pole shaped part provided to the lower end part of the guide 38 in a chain form and pads 44 are provided to the upper, under, left and right surfaces of said guide 38 and a pad 43 is provided to the tab 40B at the upper end part thereof to prevent the rotation of said guide 38. The lower end 46B of an X-frame 46 is connected in a chain form so as to surround the lower end part of the guide 38 and the frame body 40A. A pad 53 is provided to the tab 40C at the lower end of the frame 40 and support the lower end part 46B to prevent the rotation thereof. By this method, Z-, Y- and X-frames are supported so as to be made freely movable respectively to Z-, Y- and X-directions. A sensing pin 54 is provided to the lower end part 46B to allow said frames to parallelly move to arbitrary three X-, Y- and X-directions to perform measurement.