1. An image falsification detection device comprising: a decoding means for extracting, from an image of interest including a falsification detection code generated based on an original document image, the falsification detection code, and for decoding the falsification detection code to acquire falsification detection information; a difference calculation means for calculating a differential image between the falsification detection information acquired by said decoding means and said image of interest; a threshold value calculation means for calculating feature information from the differential image calculated by said difference calculation means, and for calculating a threshold value for determining whether or not a region is said falsified region corresponding to the calculated feature information; and a determination means for determining that, in the differential image calculated by said difference calculation means, a region having a difference larger than the threshold value calculated by said threshold value calculation means is the falsified region.