1. An image processing method for generating a halftone-dot image by forming a halftone dot, which is represented by a set of one or plural output dots and corresponds 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 the halftone-dot portion, the image processing method comprising: when the intensity of the image signal exceeds a predetermined value and is in a predetermined range, while maintaining contour dots, which are output dots contribute to formation of a contour of the halftone dot, to be the output dot, making a part of dots inside the contour dots to be the actual non-output dot.