A banknote is irradiated with lights of plural wavelengths. Images of the banknote for each wavelength are acquired. An IR ratio image having a pixel value that is a ratio of a pixel value of the image acquired by the visible light to a corresponding pixel value of the image acquired by the infrared light is generated. The banknote image and the IR ratio image are corrected by a coefficient corresponding to the banknote type, the banknote orientation, and the wavelength. From the banknote image or the IR ratio image, by using the information pertaining to the banknote type, the banknote orientation, and the wavelength, intermediate evaluation values are calculated for each wavelength. Mahalanobis distance is calculated based on the intermediate evaluation values, an average value and a variance-covariance matrix of the intermediate evaluation values, and a degree of soiling is determined based on the Mahalanobis distance.