PURPOSE: To stabilize an optical path of a laser interferometer for meansuring a position of a stage of a projection aligner and improve a positioning precision of the stage, by installing another fan memgber to gas control means for chamber so that a flow of gas actively generated in the circumference of the stage of the laser interferometer.

CONSTITUTION: A chamber 20 is installed so that a second condenser 6, a reticle R, a projection lens 7, a wafer W and a stage 8 are cut off from the outer atmosphere and the inside is sealed. A temperature controller 18 is disposed in the inside and also a pressure is controlled through a tube 28 by a pressure controller 26. Further, a fan 30 for forcibly generating a flow of gas in the circumference of the projection lens 7 is provided. Then, a clean gas discharged from a filter 23 is actively applied to the projection leans 7, and the flow of gas is generated in spaces S1 to S3, and thr thermal variation caused by an absorption of a part of an incident light energy in an internal leans elements L1 to L4 is reduced.