A multi-gradation image having a plurality of colors is binary coded by processing halftones in a pseudo manner. After the binary coding, an undesirable overlaying of colors on the image is eliminated so that image quality can be improved and also process load can be alleviated. A detailed process is this: Colors at a target pixel are binary coded, and the results are retained as data. When a color following the color already binary coded is processed, the retained data of the preceding color is referred to, thereby avoiding an undesirable composite black due to an overlaying of three primary colors on the same pixel. When the colors of an input image include black (K), this K is firstly binary coded at the target pixel, and when K is output, the other colors are halted outputting or a threshold value is set so that the other colors are not output. When K is not output, the other colors are binary coded in the order of higher density or higher visual sensitivity so that a quality image can be output.