An imaging optical system for an oblique incidence interferometer comprises first and second optical systems and an intermediate imaging surface therebetween. Each of the first and second imaging optical systems comprises two telecentric lenses, arranged afocal to each other, having respective focal lengths different from each other. A first image of a surface to be inspected having a deformed aspect ratio with respect to this surface is formed on the intermediate imaging surface by way of the first imaging optical system. The second imaging optical system is arranged such that the first image is focused onto the imaging surface of the interferometer as a second image corrected so as to have substantially the same aspect ratio as that of the surface to be inspected.