A gradation pattern is printed, the gradation pattern is corrected with a gamma correction table T.sub.O based on a result in which the printed gradation pattern is read, the corrected gradation pattern is printed on a photosensitive drum or the like, and a measured density of the printed gradation pattern in S4 is stored as a reference value of density D.sub.R. Thereafter, without user's operation, at a set automatic timing, the read gradation pattern P is corrected with the gamma correction table T.sub.O and printed on the photosensitive drum or the like, density deviation .DELTA.D between a measured density D of the printed gradation pattern in S6 and the reference value of density D.sub.R is determined, and the latest gamma correction table T is generated and updated on the basis of the density deviation .DELTA.D. Consequently, high-quality image formation can be automatically performed.