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, wherein the reliability calculation step comprises: an index evaluation amount calculation step of calculating evaluation amounts of the indices based on the two-dimensional geometric features and/or the three-dimensional geometric features, wherein circularities of the indices on the sensed image are acquired as the evaluation amounts of the indices detected from the sensed image, when each of the indices has a circular shape; and an index reliability calculation step of calculating the reliabilities of the indices based on the evaluation amount of the indices, wherein the reliabilities of the indices are calculated based on the circularities when each of the indices has the circular shape.