A double-conjugate maintaining optical system for maintaining the conjugate relation between an object and its image even if the distance between the object and the image varies and also maintaining another set of conjugate relation in a predetermined condition includes an afocal system comprising a plurality of lens units, a first positive lens unit disposed on the object side of the afocal system, and a second positive lens unit disposed on the image side of the afocal system. The first positive lens unit is movable relative to the second positive lens unit so that the object is positioned on the focal plane of the first positive lens unit opposite to the afocal system. The afocal system is movable along the optical axis thereof in a predetermined relation with the first positive lens unit.