1. A distortion inspection method for a projection optical system for inspecting distortion characteristics of a projection optical system to be inspected by arranging a mask formed with measurement patterns at a plurality of predetermined positions on the object surface side of said projection optical system, transferring projected images of the plurality of measurement patterns onto a photosensitive substrate arranged on the image surface side of said projection optical system, and detecting transfer images of the measurement patterns; comprising: the first step of exposing a mask, on which pairs of first and second measurement patterns are arranged adjacent to each other to be separated by a predetermined interval .DELTA.T in one direction at positions on the mask corresponding to a plurality of points at which distortion amounts are to be inspected in a projection view field of said projection optical system, onto said photosensitive substrate via said projection optical system; the second step of exposing said mask onto said photosensitive substrate via said projection optical system after said mask and said photosensitive substrate are moved relative to each other by an amount determined by the interval .DELTA.T with respect to the state in the first step; the third step of measuring relative displacements between overlapping images of the first and second measurement patterns at different image height points in the projection view field of said projection optical system; and the fourth step of calculating a value obtained by sequentially accumulating the measured relative displacements in units of image height values as a distortion amount at the corresponding image height point.