To enable the measurement of the root means squares(RMS) error of high- order front aberration in simple computation by expanding the Zernike polynomial expression on the basis of whole wave front aberration, computing the RMS errors of each aberration such as low order aberrations, astigmatism, and subtracting them from the root means square(RMS) of the whole wave front aberration.

A storage means 12 comprises a plurality of memories, and an arithmetic means 13 comprises a first to third arithmetic parts. An inputting means 11 measures the wave front aberration of an interferometer, etc., ad the first memory stores the results. The first arithmetic part computes the RMS of the whole wave front aberration W, and the third memory stores the results. The second arithmetic part expands the Zernike polynomial expression stored in the second memory to compute the magnitudes of low order aberrations, astigmatism, etc., on the basis of the whole wave front aberration W, multiples respective values by 1, 1/(2×61/2), 1/(3×81/2), and 1/(6×51/2) to compute the RMSs of each aberration, and the fourth memory stores them. Then, the third arithmetic part subtracts the RMS of each aberration from the RMS of the whole wave front aberration to compute the RMS of high order wave front aberration, and an outputting means 14 outputs the results.