PURPOSE: To simplify the entire structure of the instrument, and to maintain excellent measurement precision and secure detection performance with no probability of wrong signal generation by using variation in the pressure of compressed air as an output signal of work measurement.

CONSTITUTION: The compressed air with constant pressure is charged in an air chamber 24, and a filler 14 is pressed to before a measuring bar body 12, i.e. toward a work 10 with the internal pressure based upon the compressed air and the spring force of a pressure spring 21 and further held at a constant position while a tapered engaging surface 19 provided to ita base end part is in complete contact with a tapered bearing surface 20 provided to the tip part of the bar body 12. When the work 10 contacts a measuring sphere 11 mounted atop of the filler 14 during the measurement of the size of the work 10, an air flow from the filler support position of the bar body 12 varies in leakage according to the displacement of the filler 14, and a contact signal is outputted when the back pressure drops to a preset "threshold value".