1. An index identification method comprising: an image acquisition step, performed by an image acquisition unit, of acquiring a sensed image obtained by sensing an image of physical space in which a plurality of indices are located on a physical object; an index detection step, performed by an index detection unit, of detecting at least one index from the sensed image; a first orientation estimation step, performed by a first orientation estimation unit, of estimating an orientation of the detected index using the sensed image; an orientation information acquisition step, performed by an orientation information acquisition unit, of acquiring a measurement result of measuring an orientation of at least one of an image sensing device used to sense the image and the physical object; a second orientation estimation step, performed by a second orientation estimation unit, of estimating an orientation of at least one index based on the orientation measurement result of measuring the orientation of at least one of the image sensing device and the physical object acquired in the orientation information acquisition step, and information on the plurality of indices stored in a storage device; and an index identification step, performed by an index identification unit, of identifying to which of the indices stored in the storage device the detected index corresponds using at least the orientation of the detected at least one index estimated in the first orientation estimation step and the orientation estimated in the second orientation estimation step.