At power-on, an image forming apparatus is in a state after it is leftstanding for some time in many cases, so that an image characteristic is largely different from that when .gamma.LUT correction table is prepared by an immediately before correction and is close to that at the time of previous power-on. For this reason, image formation is judged whether it is first image formation after power-on or not (S21), and in the case of the first image formation, the .gamma.LUT correction table is replaced with an original .gamma.LUT correction table at the time of previous power-on (S22). In the case where the image formation is not the first image formation, the immediately before .gamma.LUT correction table obtained after power-on is used as it is. As a result, it becomes possible to stabilize a density or coloring of a first image after power-on while reducing a warm-up time immediately after power-on.