PURPOSE: To obtain the measuring instrument which greatly reduces measurement noise.

CONSTITUTION: The instrument which measures the size of the work has a circuit where fluid circulates. This circuit consists of a 1st branch pipe 15 equipped with an intake nozzle 16 and at least one measuring nozzle 17 which faces the work whose size is to be measured and a 2nd branch pipe 19 which is equipped with an intake nozzle 20 and open to an outlet reference nozzle 21. A pressure transducer 22 supplies an electric signal representing the pressure difference between the two branch pipes. The fluid in use is liquid and the intake nozzles are equipped with a device which supplies a liquid flow minimizing the noise of the electric signal generated by the pressure transducer to its output. Further, this device is so constituted that it can be applied for the measurement of the external diameter of a cylindrical hole, a cylindrical work, or flat surface.