Thumbnails of pages of a key document are arranged sequentially according to a page order in a row. Thumbnails of pages of a target document are arranged sequentially according to a page order in a row parallel to the row of the key document. A key page of the key document is compared with each page of the target document. When a page of the target document that is most similar to the key page is found, the row of the target document is shifted in a horizontal direction so that the page of the target document that is most similar to the key page is positioned next to the key page.