A control device arranged for use in an image forming apparatus that includes an image forming unit forming plural primary color toner images different from each other on a surface of a single latent image carrier or forming plural primary color toner images different from each other on corresponding plural latent image carriers, an image information processing unit processing image information to be input to the image forming unit based on setting values expressing tone reproduction curves which are image processing parameters forming the plural primary color toner images different from each other, and a transfer unit acquiring a superimposed color toner image by forming a transfer nip section by contacting a contact member to the surface of the single latent image carrier or each surface of the plural latent image carriers, and superimposing and transferring the plural primary color toner images formed on the single latent image carrier or the plural primary color toner images different from each other formed on the corresponding plural latent image carriers onto a surface of the contact member or a recording sheet held on the surface of the contact member, and executing control to drive the imaging forming unit and the transfer unit and a predetermined calculation process, characterised in that the control device is configured to: for a plurality of images based on image information instructed by a user, search for a predetermined color measurement region (A 1 -A 27) adapted to measure colors in the respective image indicated by the image information; measure colors (M;L*,a*,b*) of pixels of the color measurement region of a superimposed color toner image formed by the image forming apparatus based on the image information for each image; store the measured colors (M;L*,a*,b*) for each image in a sampling color database and, from the measured colors, calculate and store in the sampling color database, for each image, densities (S;c,m,y,k) proportional to area ratios of primary color toner images in the superimposed color toner image; average the measured colors and densities of the images stored in the sampling color database; perform clustering in the CMYK digital space in the sampling color database of the densities (c,m,y,k) by dividing the CMYK values into cells and determining the centres of the cells and based on said clustering determine correction amounts (δC, δM, δY, δK) corresponding to the setting values expressing the tone reproduction curves to minimize the difference, based on plural algorithms representing relationships between the measured colors previously stored in the sampling color database with respect to each of plural primary color images formed by the image forming apparatus and the setting values expressing the tone reproduction curves of the image forming unit, the averaged densities, the differences between the averaged measured colors and reference colors (R;L*,a*,b*) obtained by converting the image information and stored in the color sampling database, and current setting values expressing the tone reproduction curves; and correct the setting values expressing the tone reproduction curves based on the correction amounts. for a plurality of images based on image information instructed by a user, search for a predetermined color measurement region (A 1 -A 27) adapted to measure colors in the respective image indicated by the image information; measure colors (M;L*,a*,b*) of pixels of the color measurement region of a superimposed color toner image formed by the image forming apparatus based on the image information for each image; store the measured colors (M;L*,a*,b*) for each image in a sampling color database and, from the measured colors, calculate and store in the sampling color database, for each image, densities (S;c,m,y,k) proportional to area ratios of primary color toner images in the superimposed color toner image; average the measured colors and densities of the images stored in the sampling color database; perform clustering in the CMYK digital space in the sampling color database of the densities (c,m,y,k) by dividing the CMYK values into cells and determining the centres of the cells and based on said clustering determine correction amounts (δC, δM, δY, δK) corresponding to the setting values expressing the tone reproduction curves to minimize the difference, based on plural algorithms representing relationships between the measured colors previously stored in the sampling color database with respect to each of plural primary color images formed by the image forming apparatus and the setting values expressing the tone reproduction curves of the image forming unit, the averaged densities, the differences between the averaged measured colors and reference colors (R;L*,a*,b*) obtained by converting the image information and stored in the color sampling database, and current setting values expressing the tone reproduction curves; and correct the setting values expressing the tone reproduction curves based on the correction amounts.