When generating one file from image data having plural pages, the present invention generates coded data of a background portion that is common to plural pages for sharing the coded data, thereby realizing a file compressed at high compression rate while maintaining image quality. For this, the apparatus reads plural pages. In the page processing, the read page image is separated into a text image and a background image. The background image is divided into predetermined pixel blocks, then comparison is performed with respect to a certain position between one page and a previous page in units of pixel block, and common pixel blocks are separated from page-unique pixel blocks. When reading of all pages is completed, a page image is generated with the text image and the page-unique background image. Thereafter, the background image common to plural pages is linked to corresponding pages for generating one file.