PURPOSE: To improve the picture quality of an image after color subtraction by applying a pseudo gradation technique to a color image and to perform the color subtraction in a short processing time by providing M colors of the color subtraction in a fixed state.

CONSTITUTION: This device is provided with a CPU (processing means) 2 controlling the whole device 1, and is constituted so as to connect respectively a cache memory 3, an HDD 5 as a color image storage means through an HDD(Hard Disk Drive) controller 4, a dither memory 6, a display device 8 as an output means such as an LCD (liquid crystal display device), etc., through a display controller 7 and a key board 10 as a selection means through a key board controller 9 to the CPU 2. Then, after the CPU 2 performs the simple color reduction to the color image, it performs the color subtraction from M colors to N colors by processing pseudo gradation. Thus, the picture quality is improved compared with the image pseudo gradation expressed by a regular system, and moreover, the color subtraction is performed in a shorter processing time compared with a method performing the color subtraction by judging color distribution.