A method and apparatus authenticating a bill. The method irradiates infrared light having a predetermined wavelength onto a print area of a genuine bill from a light emitting unit, stores transmitted light data of light transmitted through the genuine bill as reference data, irradiates infrared light having the predetermined wavelength onto a print area of the bill to be authenticated from the light emitting unit, and compares transmitted light data of infrared light transmitted through the bill with the reference data. The method further determines in advance a region different in visibility under visible light and under infrared light as a specific region in a print area of the bill, applies a predetermined weighting to the transmitted light data of light in the specific regions of the bill to be authenticated and the genuine bill, and compares the weighted data with each other. Based on comparison results the bill is authenticated.