A printer divides an input image into a plurality of blocks and extracts a block image corresponding to each of the blocks. The colors of the block image are reduced to, for example, four colors. Index numbers are given to pixels of the color-reduced image, and the four colors are assigned. To a color palette in which color information of the four colors is described in correspondence with the index numbers, object information indicative of the kinds of images (for example, a character, a chart, and a photographic image) is added. Consequently, the handling performance of object information for use in an image processing such as a color conversion process is improved.