A vacuum optical system has a vacuum chamber for housing an optical system used in a vacuum. The vacuum chamber is equipped with a member, on which the optical system is at least mounted, supported by such parts that when a pressure in the vacuum chamber changes to deform the vacuum chamber, the amount of displacement transmitted to the optical system is smaller than a predetermined tolerance depending on an accuracy necessary for the optical system. Thus, the vacuum optical system can be obtained which does away with the need for readjustment of optical alignment and is compact.