1. An image processing method for generating a halftone image by forming a halftone dot represented by a set of one or plural output dots corresponding to an intensity of an input image signal while making a part of the dots constituting the halftone dot to be an actual non-output dot so as to reduce an amount of a coloring material of a halftone-dot portion, the image processing method comprising: holding in a predetermined storage medium halftone-dot profile data for forming the halftone dot corresponding to the intensity of the image signal and gap-size profile data representing a size of a set of the non-output dot; generating the halftone dot comprising a gap having a size based on the gap-size profile data, a size of the halftone dot being based on the halftone-dot profile data; and forming the halftone dot having the gap with the reduced amount of the coloring material of the halftone-dot portion and reducing coloring material accumulation when forming the halftone dot, the coloring material being reduced when compared with an amount of a coloring material used to form a halftone dot comprising no gap for the same intensity of the image signal, wherein the gap is created by making the part of the dots constituting the halftone dot to be the actual non-output dot.