1. An X-ray image processing method of estimating a position and attitude of an measuring object by comparing the measuring object on an X-ray projection image with a parent database storing data of a plurality of candidate objects, wherein said measuring object is an artificial joint bone or a bone of living tissue, and the 3D shape of said candidate object is known, said parent database stores an element-contour group of said plurality of candidate objects projected on 2D plane from various angles, a part of a contour of said measuring object is deficient, said method comprising: a step of calculating a start point and an end point of the contour other than the deficient part of said measuring object; a step of estimating a start point and an end point of each element-contour of said element-contour group corresponding to the start point and the end point of the contour of said measuring object; a step of estimating an element-contour corresponding to the contour of said measuring object and a candidate object corresponding to said measuring object by comparing a coefficient of open curve Fourier descriptors calculated from the contour, said start point and end point of said measuring object with a coefficient of open curve Fourier descriptors calculated from the contour, said start point and end point of each element-contour of said element-contour group; and a step of calculating a position and attitude of said measuring object based on the element-contour corresponding to the contour of said measuring object and the candidate object corresponding to said measuring object.