1. A position and orientation measurement method, comprising: an input step of inputting sensed images of respective frames time-serially sensed by an image sensing device; a detection step of detecting indices allocated or set on an object from each of the sensed images; an acquisition step of acquiring, in association with each of the indices detected in the sensed image of a frame of interest, a detection condition from frames before the frame of interest; a reliability calculation step of calculating a reliability of the index according to the detection condition of that index acquired in the acquisition step; and a position and orientation calculation step of calculating a position and orientation of the object or the image sensing device using at least information associated with image coordinates of the respective indices detected in the detection step and the reliabilities of the indices calculated in the reliability calculation step.