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 comprising to: means for creating an initial quantizing pattern to determine a dot occurrence sequence for an initial level value among level values of the multi-density level image; means for creating a plurality of quantized patterns for each level value of the multi-density level image to determine a dot occurrence information sequence, based on a preceding quantized pattern which has been created, such that anti-correlation is increased with respect to the dot occurrence information sequence for a pixel of the level value that has already been quantized; means for adding the plurality of quantized patterns corresponding to all the level values of the multi-density level image to form a dither mask; and means for quantizing the multi-density level image data by using the dither mask.