Method for discriminating authenticity of a bill using polarization and an apparatus therefor

An apparatus and method for discriminating the authenticity of a bill (1) put into an automatic vending machine, etc. on the basis of a degree of brilliance of a surface of the bill, comprises two photodiodes (3a, 3b) for receiving light irradiated from a light-emitting diode (2) and reflected by the bill, and a polaroid filter (4) disposed in front of one of the photodiodes. A differential amplifier (6) or a divider (B) is supplied with output currents from the photodiodes, respectively representing an amount of the reflected light received not through the polaroid filter and containing a polarized component which is produced when the light is reflected on the surface of the bill, and an amount of the reflected light, received through the filter and containing no polarized component, and generates an output voltage indicative of the difference between or the ratio of the output currents, which voltage is compared with a corresponding reference voltage in a comparator (7) whose output assumes a Low-level for a lusterless authentic bill, and a High-level for a brilliant counterfeit bill obtained by color-copying the authentic bill, whereby the authenticity of the bill is accurately discriminated.