1. A method of detecting a displacement of an observed image with respect to a reference image in vertical and lateral directions, comprising the steps of: (a) obtaining a vertical cumulative projection pattern with respect to the vertical direction for each of the observed image and the reference image, and a lateral cumulative projection pattern with respect to the lateral direction for each of the observed image and the reference image; (b) obtaining candidate vertical displacements .DELTA.x of the observed image with respect to the reference image in the vertical direction for which a vertical discrepancy between the vertical cumulative projection patterns for the observed image and the reference image is locally minimum, and candidate lateral displacements .DELTA.y of the observed image with respect to the reference image in the lateral direction for which a lateral discrepancy between the lateral cumulative projection patterns for the observed image and the reference image is locally minimum; (c) obtaining an oblique cumulative projection pattern with respect to an oblique direction for each of the observed image and the reference image; (d) calculating an estimated oblique displacement .DELTA.z from each of the candidate vertical displacements .DELTA.x and each of the candidate lateral displacements .DELTA.y obtained at the step (b); (e) adjusting the oblique cumulative projection pattern for the observed image by the estimated oblique displacement .DELTA.z calculated at the step (d) to obtain an adjusted oblique cumulative projection pattern; (f) obtaining an oblique discrepancy between the oblique cumulative projection pattern for the reference image and the adjusted oblique cumulative projection pattern for the observed image; and (g) determining the displacement in the vertical and lateral directions as one of the candidate vertical displacements .DELTA.x and one of the candidate lateral displacements .DELTA.y for which the oblique discrepancy obtained at the step (f) satisfies a prescribed condition.