1. An information processing method comprising: acquiring a sense image of an image sensing unit; acquiring coordinate values of registered indices; detecting an image position of an additional index in the sense image in accordance with a user's instruction; detecting image positions of the registered indices in the sense image; calculating a coordinate value of the additional index using the image positions of the registered indices, the coordinate values of the registered indices, and the image position of the additional index; and registering the additional index as a registered index using the calculated coordinate value.