1. An image forming apparatus that uses a table generated in advance to correct a gradation characteristic of an image to be formed when carrying out image forming using an image carrier, comprising: a patch forming unit configured to form a patch in an area where an image is not formed on the image carrier; a measuring unit configured to measure a density of the patch formed on the image carrier; and a modifying unit configured to modify the table based on the density of the patch measured by the measuring unit, wherein the patch forming unit forms the patch concurrent with image forming to the image carrier.