A method of analyzing the astigmatic difference of an optical device for analyzing the astigmatism inherent to the optical device by utilizing a condensation performance analysis system while correcting the astigmatic difference inherent to the condensation performance analysis system for analyzing the condensation performance of the optical device to be analyzed. The method of analyzing an astigmatic difference has steps comprising: obtaining the astigmatic difference R(.theta..sub.n) at an angle .theta..sub.n while rotating the data points obtained from the optical device about given basic axes; computing a vector coefficient R.sub.0 in the same direction as the basic axes and another vector coefficient R.sub.45 at 45 degree to the basic axes by utilizing the maximum angle .theta..sub.max at the greatest value of the astigmatic difference R(.theta..sub.n) and the greatest astigmatic difference .theta..sub.max; computing an astigmatic difference R(.theta.) at an arbitrary angle .theta. derived from the vector coefficient R.sub.0 and the vector coefficient R.sub.45 by a fitting technique; correcting the vector coefficient R.sub.0 and the vector coefficient R.sub.45 by analyzing the condensation performance analysis system using a reference optical device with a known astigmatic difference; and computing the astigmatic difference of the optical device by utilizing the corrected vector coefficient R.sub.0 and corrected vector coefficient R.sub.45.