1. A paper medium information encrypting system comprising: a digital image acquiring unit configured to acquire a digital image of information by capturing an image of a paper medium on which the information is recorded; an encryption area designating unit configured to designate an encryption target partial area in the digital image acquired by said digital image acquiring unit; an image converting unit configured to convert the image of the partial area into a processed image on the basis of an encryption key; a pixel value converting unit configured to generate a converted image having regularity, used for specifying a position of the partial area on the occasion of decryption, by regularly converting pixel values of the processed image; and an output unit configured to output the encrypted image containing the converted image, wherein said encryption area designating unit designates, as the encryption target partial area, an area specified by the area designating information for specifying the area on the digital image.