1. A calibration data creating device comprising: a reference measured density memory storing, as reference measured densities, measured densities of reference density patches that are formed by an image-forming unit based on a single group of reference density patch data, the single group of reference patch data including a plurality of sets of reference density patch data for a plurality of reference set densities, the reference density patch data being binarized image data; a target density memory storing for target densities in association with a plurality of set densities; a dither data memory for storing a plurality of sets of dither data, each set of dither data that has a plurality of dot locations and that is used for creating binarized image data based on each set density, the dither data having a prescribed threshold at each dot location; a corresponding set density determining unit determining, for each set of dither data, corresponding set densities for the plurality of the reference set densities being used for creating, based on the subject set of dither data, binarized image data that enables the image-forming unit to form an image whose density corresponds to the measured density of the reference density patch that is formed by the image-forming unit based on the reference density patch data at the each reference set density; a calibration data creating unit creating calibration data indicating a correlation between the set densities and calibrated set densities, the calibrated set density for each set density being used for creating, based on the subject set of dither data, binarized image data that enables the image-forming unit to form an image whose density is the same as the target density for the each set density, the calibration data creating unit creating the calibrated set densities based on the target densities, the reference measured densities, and the corresponding set densities; and a calibration data memory storing the calibration data.