The present invention provides an image processing apparatus on which an image forming engine for forming a full-color image by using color materials corresponding to data of color tones on the basis of C (cyan), M (magenta), Y (yellow), and K (black) data is mounted. The image processing apparatus includes: a color converting component for converting R (red), G (green), and B (blue) data input from the outside to C.sub.(1), M.sub.(1), and Y.sub.(1) data which depends on a characteristic of the image forming engine; a lightness acquiring component for acquiring lightness from the C.sub.(1), M.sub.(1), and Y.sub.(1) data obtained by the conversion of the color converting component; a memory for storing a lightness-gray value characteristic table; a gray value reading component; a first arithmetic component for obtaining C.sub.(2), M.sub.(2), and Y.sub.(2) data; a K data generating component; and a second arithmetic component for calculating C, M, and Y.