The present invention provides a projection optical system which projects an image on a first object plane onto a second object plane, comprising a plurality of optical members inserted in turn from a side of the second object plane, said plurality of optical members being made of an isotropic crystal and including a first optical member and second optical member in each of which <1 1 1> crystal axes are oriented in a direction of an optical axis and a third optical member in which <1 0 0> crystal axes are oriented in the direction of the optical axis, wherein maximum angles .theta.1, .theta.2, and .theta.3 between the optical axis and light beams passing through said first optical member, said second optical member, and said third optical member, respectively, satisfy |.theta.i-.theta.j|<5.degree. (i, j=1, 2, 3).