Image data of a pixel of interest is inputted, diffusion data diffused from errors occurred by decrease of a tonal number of image data is added to the image data of the pixel of interest, the tonal number of the image data of the pixel of interest, to which the diffusion data has been added, is decreased using an error diffusion method or minimized average error method, and an error occurred in the decrease of the tonal number for the pixel of interest is diffused to pixels close to the pixel of interest. Further, the errors occurred by the decrease of the tonal number of the image data in a predetermined image region are integrated as an integrated error, and a threshold value used in the error diffusion method or minimized average error method is set based on the image data of the pixel of interest and the integrated error.