1. An image forming apparatus comprising: a plurality of image forming units, one image forming unit being provided for each of a plurality of colors, and forming an image of the respective corresponding color; a density detecting unit configured to detect a density of density detecting patches; a processor; memory having executable instructions stored thereon that, when executed by the processing unit, cause the processor to operate as a patch forming unit, which actuates the respective image forming units according to image forming conditions for the respective colors of the image forming units to form the density detection patches of the respective colors; a condition correcting unit that controls the density detecting unit to detect the density of the density detecting patches of the respective colors, and corrects the image forming conditions based on respective deviations between the detected densities of the density detecting patches of the respective colors and target densities defined for each of the colors; a patch re-forming unit that, for each color for which the respective deviation is greater than a threshold value, actuates the image forming unit corresponding to the color so as to generate a re-formed density detecting patch of the color using the respective image forming condition previously corrected by the condition correcting unit; a condition re-correcting unit that is configured to control the density detecting unit to detect a density of the re-formed density detecting patch and re-correct the respective image forming condition of the color of the re-formed density detecting patch based on a deviation between the density of the re-formed density detecting patch and the target density of the color corresponding to the re-formed density detecting patch; a preparation process unit that executes a preparation process to prepare for detection of the density by the density detecting unit before the density detecting patch is formed by the patch forming unit; and a preparation process re-executing unit that re-executes the preparation process before the re-formed density detecting patch is generated by the patch re-forming unit; and a cleaning unit that cleans the density detecting patch formed by the patch forming unit, wherein the preparation process re-executing unit executes the preparation process at a same time that the density detecting patch is cleaned by the cleaning unit.