Disclosed is an optical system having a plurality of optical elements, for use in an optical instrument or a projection exposure apparatus, arranged to maintain a best optical performance by controlling a relative position of the plurality of optical elements. The optical system includes a reference mirror, a first measuring system for measuring a relative position, with respect to the reference mirror, of a first optical element among the plurality of optical elements and/or a first target mirror disposed so that a relative position thereof with respect to the first optical element is kept substantially unchanged, and a second measuring system for measuring a relative position, with respect to the reference mirror, of a second optical element among the plurality of optical elements and/or a second target mirror disposed so that a relative position thereof with respect to the second optical element is kept substantially unchanged. At least one of the first and second optical elements is moved on the basis of the measurement so that a relative positional relationship between the first and second optical elements is placed in predetermined positional relationship.