Objects of the present invention are to facilitate filling out by hand an electronic paper and to realize digitization of contents with which the electronic paper is filled out by hand with a simple structure. A read image is obtained by optically reading a reading-object of the electronic paper. Based on the document information written on the electronic paper and writing-conditions during document writing, which are represented by the document information, a display image of the document in accordance with the read image information is formed. A written image is separated and extracted by converting a density only of pixels among the respective pixels of the read image, whose densities vary in accordance with the document writing and which correspond to a portion, to which ink of a pen for the handwriting is not adhered.