A bill processing apparatus capable of conducting accurately an authenticity judgment of a bill, which is caused to expand or contract, and an authenticity judgment process method employed therein. The bill processing apparatus includes a bill reader reading a bill, a reference data storage part storing an allowable range acceptable from a reference value as a standard length of a printed area on each surface thereof; and a comparison judgment part executing an authenticity judgment process by comparing the corrected data with an allowable ranged for the other surface stored in the storage part, the actual measurement data on the other surface being corrected based on a correction value calculated with respect to the actual measurement data for the one surface, when the actual measurement data of the length of the printed area of the one surface is out of the allowable range.