1. An index identification method for detecting and identifying an index from an acquired image using position information and geometric information of the index, which are held in advance, the method comprising: using a computer to perform the steps of: an image acquisition step of acquiring an image obtained by sensing a real space including an index having a predetermined shape by an image sensing device; a position and orientation information acquisition step of acquiring measurement results of a three-dimensional position and orientation of at least one of the image sensing device and the real object; a detection step of detecting the index from the acquired image and obtaining position information and geometric information of the detected index; an estimation step of calculating estimated position and geometric information of the index on an image sensing plane of the image sensing device on the basis of the three-dimensional position and orientation measurement results and the position information and geometric information of the index held in advance; and an index identification step of identifying the detected index by comparing the obtained position information and geometric information of the detected index with the estimated position and geometric information of the index, wherein the geometric information of the index held in advance indicates a directionality of the index in a plane where the index is arranged.