1. A method of creating a threshold value matrix used for quantization of n-value signal in each pixel into m-value (n>m>1) using a dither process, by a computation device, the method comprising: executing a first process of creating an initial dot pattern of a predetermined dot rate “a” and arranging a gradation value corresponding to the dot rate “a” at a pixel position of the threshold value matrix, the pixel position corresponding to the initial dot pattern, as a threshold value; executing a second process of one of i) creating a next dot pattern of dot rate “b” that is a dot rate increased by generating new dots including dots having a dot pattern of dot rate “b” based on an error diffusion method and in doing so, arranging a gradation value corresponding to the dot rate “b′” as a threshold value in a pixel position of the threshold value matrix corresponding to a position for generating each of the new dots, and ii) creating a next dot patter of dot rate “b′” that is a dot rate decreased by removing any of the dots in the dot pattern having the dot rate “b” based on an error diffusion method; and in doing so, arranging a gradation value corresponding to the dot rate “b′” as a threshold value in a pixel position of the threshold value matrix corresponding to a position for removing any of the dots; and repeating the second process for each dot rate, wherein the initial dot pattern or the dot pattern generated in the second process having been executed one step before is used as a dot pattern of dot rate “b”.