A page numbering unit assigns an electronic document page number to each of image data of a plurality of pages stored in a storage unit. An image analysis unit extracts a page number described in each of the image data of the plurality of pages stored in the storage unit. The image analysis unit identifies image data that describes page numbers for searching for other pages, from among the image data of the plurality of pages stored in the storage unit. A page number comparator compares the assigned electronic document page number with the extracted page number, for each of the image data of the plurality of pages. A page number conversion unit converts the page numbers for searching into the corresponding electronic document page numbers, based on a result of comparison by the page number comparator.