A falsification detection threshold value for falsification detection is automatically determined. A falsification detection portion (4) performs division into uniform detection units, and calculates, for each of the cells of the detection units, the variation information of differential information on respective regions that are included in the detection units and are divided as described above (S802). It is understood that cells (901), (902) and (903) have values of 8, 170 and 140, respectively. The falsification detection portion (4) uses the variations in the differential information calculated in step S802 to calculate a differential information threshold for determining a falsified region within an image read by scanning (S803). The average value and the standard deviation of a region where falsification is not conducted are relatively small, and the average value and the standard deviation of a region where falsification is conducted are relatively large.