It is difficult to appropriate estimate the colorimetric values of an arbitrary patch at a desired temperature with respect to an arbitrary medium in consideration of the influence of a fluorescent whitening agent contained in the medium. Target patch colorimetric values obtained when patches of a plurality of colors formed on a medium using color samples are measured at a target temperature designated in step S101. This estimation is implemented when both media-dependent variation correction processing (S102) that estimates patch colorimetric values based on the temperature characteristics of the medium, and toner-dependent variation correction processing (S103) that estimates patch colorimetric values based on the temperature characteristics of the color samples are executed. By correcting image data whose image is to be formed based on the target patch colorimetric values (S104), an arbitrary color patch on an arbitrary medium can be coped with.