1. An image processing apparatus connected to a plurality of image sensing devices each of which senses physical space where a plurality of indices are arranged, comprising: a calculation unit which, for each image sensing device, recognizes an index in a sensed image and calculates layout information of the recognized index in a coordinate system based on an image sensing device that has acquired the sensed image; a management unit which manages index information including identification information unique to the index and the layout information of the index; and a varying unit which, 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, varies the index information of the first index managed by said management unit on the basis of the layout information of the first index calculated by said calculation unit for a second sensed image acquired by a second image sensing device other than the first image sensing device.