1. An information processing method for an information processing apparatus comprising the steps of: acquiring a sense image of an image sensing unit with the information processing apparatus; acquiring coordinate values of registered indices with the information processing apparatus; detecting image positions of the registered indices in the sense image with the information processing apparatus; calculating position and orientation of the image sensing unit on the basis of the detected image positions of the registered indices and the coordinate values of the registered indices with the information processing apparatus; detecting an image position of an additional index in the sense image in accordance with a user's instruction with the information processing apparatus; calculating a coordinate value of the additional index using the image positions of the registered indices, the coordinate values of the registered indices, the position and orientation of the image sensing unit, and the image position of the additional index with the information processing apparatus; and registering the calculated coordinate value of the additional index as a registered index with the information processing apparatus, wherein when the coordinate value of the additional index cannot be calculated due to insufficient information for calculating the coordinate value, the additional index is temporarily registered as a vague index, wherein when the coordinate value of the additional index can be calculated due to sufficient information for calculating the coordinate value, the additional index is regularly registered as a regular index, and wherein when the coordinate value of the vague index can be calculated due to sufficient information for calculating the coordinate value, the vague index is re-registered as a regular index.