PURPOSE: To perform the correction of fluctuations due to temperature and atmospheric pressure in a real time by a system wherein a second laser interferometer for correction is set on the base of a positioning device, in addition to a laser interferometer for positioning, and the data obtained from the second laser interferometer are incorporated into a part of a closed loop servo system of the positioning device.

CONSTITUTION: A main base 1 supporting the whole of a positioning device is made generally of granite or the like. An X-Y state 2 is moved on the main base 1 in the directions indicated by arrows 3 and 4, and the position thereof is measured from the relative position between interrferometers 17, 18, 19 and a reflector 20 and determined thereby exactly. A second laser interferometer 16 is fixed firmly on the main base 1, measuring constantly an optical path running between the interferometer 16 and a reflector 25. An output of each interferometer is transduced into an electric signal, counted by a counter, sent to a control unit for positioning a movement, and incorporated into a closed loop circuit. The amount of correction due to the fluctuations of temperature, atmospheric pressure and humidity, etc. is computed by a correction and a subtractor of the closed loop circuit, and thus the correction is performed in a real time.