1. A marker generation device comprising: a video image inputting unit configured to input at least one background image; a measuring unit configured to measure, based upon the background image, postural changes of an object within this image; a feature extracting unit configured to extract feature points from said inputted background images; an invariant feature converter configured to display the feature points extracted from said at least one background image onto an invariant feature space with a predetermined conversion process responding to said postural changes; a unique feature selector configured to select as a unique feature a portion of said invariant feature space in which said feature point does not appear; and a marker generating means configured to generate a marker by employing said unique features: wherein said measuring unit comprises a basis number setting unit configured to measure the kinds of the postural changes of the object within this image based upon said inputted background images, and decide the number of corresponding reference points responding to said kinds of the postural changes; wherein: said invariant feature converter is configured to display, as said predetermined conversion process, each feature point extracted from said at least one background image onto the invariant feature space with a mapping process employing said decided number of reference points; said unique feature selector is configured to select as the unique feature a portion of said invariant feature space in which said feature point does not appear; and said marker generator is configured to generate the marker by employing said unique features.