The technique of the present invention enhances the stability of paper material identification of sheets. To achieve the above purpose, both short-wavelength light in the range of 370 nm and long-wavelength light in the range of 420 to 1000 nm are irradiated to paper to be identified in identifying the paper material. The identification is carried out, based on the difference in absorbance of the paper, which is obtained for each irradiated light. The absorbance of the paper varies according to the paper material, thereby enabling the identification of the paper material free from influence, which are caused by differences in manufacturing process, such as shading patterns. In addition, the simultaneous use of the short-wavelength light and the long-wavelength light declines influence on the absorbance, which are caused by environmental factors, such as humidity and deterioration of sheets, thereby resulting in stable identification of the paper material.