1. A position and orientation measuring method for measuring a position and an orientation of a first imaging apparatus and a position and an orientation of at least one object body observed by the first imaging apparatus, the position and orientation measuring method comprising: (a) detecting an index provided on the observed object body, based on a first image obtained from the first imaging apparatus; (b) detecting an index provided on the observed object body, based on a second image obtained from a second imaging apparatus; (c) detecting an index provided on the first imaging apparatus, based on the second image; and (d) contemporaneously calculating a position and an orientation of the first imaging apparatus and a position and an orientation of the observed object body, based on information relating to image coordinates of the indices detected in (a), (b) and (c).