Error diffusion processing is performed for each pixel in each region of an image divided into a plurality of regions, by scanning in both the first and second directions different to each other. A diffusion coefficient set for diffusing, to the pixel of interest to be processed, a quantization error generated upon quantizing a pixel near the pixel of interest is set in accordance with the position of the pixel of interest in scanning in the first direction in the region of interest. The pixel of interest to which an error has been diffused from a pixel position referring to the set diffusion coefficient set is quantized.