1. A distribution curve generating device for generating a distribution curve for an image forming device, comprising: a first distribution curve storing portion storing a plurality of first distribution curves, each being used for distributing an original color signal of a predetermined color into a color signal for normal ink of the predetermined color and a color signal for light ink of the predetermined color, the light ink having density lighter than the normal ink; a color signal inputting portion inputting a plurality of original color signals indicative of an image; a density data detecting portion processing the original color signals inputted by the color signal inputting portion based on at least one of the plurality of first distribution curves stored by the first distribution curve storing portion and detecting the density data of the image that is produced by an image forming device according to the processed original color signals; and a second distribution curve generating portion generating at least one second distribution curve based on: the at least one first distribution curve, the density data detected by the density data detecting portion, and the original color signals inputted by the color signal inputting portion.