1. A position and orientation measurement method comprising: an image input step of inputting an image sensed by an image sensing device; an index detection step of detecting indices allocated or set on an object from the sensed image; a reliability calculation step of calculating respective reliabilities of the indices based on two-dimensional geometric features of the indices on the sensed image and/or three-dimensional geometric features that represent relationships between the image sensing device and the indices on a three-dimensional space; and a position and orientation calculation step of calculating a position and orientation of the object or the image sensing device using the reliabilities of the indices and information associated with image coordinates of the indices.