PURPOSE: To automate the exchange of a head, in a measuring system using an air micrometer.

CONSTITUTION: A device comprises an air micrometer having an air ejection head holding part 3 wherein a plurality of different air ejection heads 4a, 4b, 4c for air micrometer are removably installed, and an air micrometer main body part 1, an air micrometer head moving mechanism 5 for moving the air ejection head holding part 3, a head housing mechanism 6 for housing a plurality of air ejection heads in the state capable of being exchanged for either of a state axially movable and a fixed state, and housing head moving mechanisms 7-9 for moving the head housing mechanism 6 so that the head is at a head exchange position in a head exchange time and for a shunting position in a measuring time, and is a measuring system using the air micrometer making the head automatically exchangeable.