1. An image processing apparatus for processing image data having a plurality of pixels and expressing an image using a gradation value for each pixel, the image processing apparatus comprising: an input unit for inputting the image data; a region-dividing processor for dividing the image into at least a first region and a second region on the basis of the inputted image data; a comparator for comparing gradation values of pixels in the first region and threshold values corresponding to the pixels of the first region of a dither mask having a plurality of threshold values; a first dot-decision processor for deciding whether or not dots are formed in pixels of the first region among first boundary pixels, which are pixels in the first region near the boundary between the first region and the second region, using information on pixels included in the first region and not using information on pixels included in the second region; a second dot-decision processor for deciding whether or not dots are formed in pixels of the second region among some of the pixels in the second region, using information on pixels included in the first region and information on pixels included in the second region; and a dot data generator for generating dot data for the image data using formation or non-formation of dots as decided by the first and second dot-decision processors.