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 area designating information acquiring unit configured to acquire an area designating information on the basis of the information recorded on the paper medium; 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 a regularity at a spaced interval, used for specifying a position of the partial area when performing decryption, by adding a predetermined value to pixel values of the processed image according to a checkered pattern, without increasing a number of the pixels; and an output unit configured to output an encrypted image containing the converted image having the regularity at the spaced interval, wherein said encryption area designating unit designates, as the encryption target partial area, an area specified by the area designating information for specifying the encryption target partial area on the digital image.