1 : A marker generation device comprising: a measuring unit that measures, based upon a background image, postural changes of an object within this image; an invariant feature converter that displays feature points extracted from said background images onto an invariant feature space with a predetermined conversion process responding to said postural changes; a unique feature selector that selects as a unique feature a portion of said invariant feature space in which said feature point does not appear; and a marker generating means that generates a marker by employing said unique features.