Image processing method and device and image formation device

In a multilevel error diffusion process, an input image signal Din with (256) tones and an image correction signal Ecd are added together to produce a corrected image signal Dinc, which in turn is converted into an eight-level image signal to provide an output image signal Dout with eight tones. To convert the input image signal into the output image signal, there are provided a plurality of conversion tables (51 to 55) and a switching means (56) for selectively switching the conversion tables at random. The conversion tables (51 to 55) are switched with each pixel in the corrected image signal.