For each image sensing device, an index in a sensed image is recognized, and layout information of the recognized index in a coordinate system based on an image sensing device that has acquired the sensed image is calculated. Index information including identification information unique to the index and the layout information of the index is managed. If recognition of a first index in a first sensed image acquired by a first image sensing device has failed, or the first index has erroneously been recognized, the index information of the first index is varied on the basis of the layout information of the first index calculated by the above process for a second sensed image acquired by a second image sensing device other than the first image sensing device.