1. An image processing apparatus comprising an arithmetic processing section for quantizing multi-density level image data and converting the image data into digital halftone image data for outputting, the arithmetic processing section being configured to: create a plurality of quantized patterns to determine a dot occurrence information sequence, based on a given initial quantized pattern, such that anti-correlation is increased with respect to the dot occurrence information sequence for a pixel that has already been quantized; add the plurality of quantized patterns to form a dither mask; and quantize the multi-density level image data by using the dither mask.