When an image forming apparatus sequentially reads the document images of a plurality of pages, forms images on sheets and outputs the sheets (aggregation mode and similar), the image forming apparatus compares, for partial data of the data for a plurality of pages generated by a reading device, coded partial data after first image processing with coded partial data after second image processing. If the coded partial data after first image processing is smaller, the coded data for the plurality of pages after first image processing is caused to be stored in a compressed data storage portion. Then, a first coding/decoding portion is caused to read the stored data, a second image processing portion and similar are caused to perform processing of the data, and a printer is caused to form images on sheets and output the sheets.