PURPOSE: To attain gamma correction corresponding to diversified change of a gradation characteristic accurately by allowing a gamma compensation means to revise the correction content of a gamma correction means based on the content of 1st and 2nd storage means.

CONSTITUTION: A picture data D4 is inputted to a gamma correction section 26 and stored in a 1st memory 28 or a 2nd memory 29 according to the control by a CPU 201 in the gamma compensation processing. The CPU 201 fetches an output into its inside in response to each density level in a density conversion table NT at that point of time and the storage content of the memories 28,29 and obtains a new output in response to each density level in the density conversion table NT based on the data. Thus, the device copes with diversified change in the image forming characteristic of an image forming system 40 to reproduce the density of a picture of an original correctly.