Patent ID: 8854186

Claim:
A sheet processing apparatus comprising: an insertion slot into which a sheet is inserted, the sheet including a printed area and a non-printed area on each side of the sheet; a conveyor member which conveys the sheet inserted into the insertion slot; a sheet reading unit which reads the sheet conveyed by the conveyor member; a detection unit which detects an actual measurement value of length in the printed area along a traveling direction of the sheet being read by the sheet reading unit; a storage part which stores a reference value of length in the printed area of the sheet; and an authenticity judgment part which judges an authenticity of the sheet based on a comparison result of the actual measurement value and the reference value, wherein the reference value is determined based on a calculated length as the length in the printed area is calculated by reading a legitimate sheet; a theoretical value of length of the printed area of the legitimate sheet; and a tolerance as an acceptable value in a legitimate sheet with respect to the theoretical value, and the reference value is changed by using the legitimate sheet as a base and utilizing the actual measurement value of length of the legitimate sheet to achieve a new reference value inherent in the processing apparatus.