1. A three-dimensional position/orientation sensing apparatus for measuring a relative three-dimensional position/orientation relation between an object and a capturing apparatus from an image captured by the capturing apparatus, said sensing apparatus comprising: an image input section which inputs the image obtained by capturing said object and a marker having a known three-dimensional position/orientation relation by said capturing apparatus; an identifying section which uses a geometric characteristic of the marker to identify said marker in the image inputted from said image input section; and a position/orientation detecting section by a single marker, which analyzes the image of one of a plurality of markers identified by said identifying section, obtains a relative position/orientation relation between the marker and said capturing apparatus, and obtains the three-dimensional position/orientation relation between said object and said capturing apparatus.