A personal computer is operable to sectionalize a document image to form a plurality of image areas each having an attribute associated with a compression mode. Then, it is determined whether the image areas overlap with one another to form an overlap area. When the overlap area is formed, the overlap area is separated from the engaged image areas. If one of the engaged image areas is included within the remaining image areas, an attribute for the overlap area is set up according to the inclusive relationship of the engaged image areas. If there is not such an inclusive relationship, the attribute in the overlap area is set up according to the occupancy rate of the overlap area to each of the engaged image areas, and weighting coefficients of the respective attributes of the engaged image areas.