A character is recognized from an original document image that is obtained, for example, by an image reading apparatus. And a natural language processing is performed on a document configured from the recognized characters. Thus, a translation (supplementary annotation) for a word or a phrase in the document is obtained. Then, a supplementary annotation added document image is generated with an original document image layer configured from an original document image on which a supplementary annotation text layer is superimposed. In the supplementary annotation text layer, the translation is placed at a position corresponding to a position in an interline space near the word or the phrase. Furthermore, in addition to a translation, an underline is placed for a discontinuous phrase.