An optical system deviation estimating apparatus includes an erected/inverted attitude setting means 9 for changing dispositional attitude of an optical system under test, a non-interferometric type wavefront measuring means 10 for measuring wavefronts at the attitudes as set up without resorting to interference phenomenon of light, a polynomial approximation means 15 for expanding measured wavefront values determined by the non-interferometric type wavefront measuring means 10 to a polynomial, an averaging arithmetic means 11 for averaging the measured values derived from output of the non-interferometric type wavefront measuring means 10 or alternatively arithmetic values derived from output of the polynomial approximation means 15, and a polynomial specific coefficient extraction arithmetic means 16 for extracting specific coefficient values of the polynomial.