1. An image forming apparatus comprising: density correcting means for correcting densities of respective pixels of an input image using a density correcting parameter; forming means for forming an image, in which densities of respective pixels of the input image have been corrected by said density correcting means, on a sheet; changing means for changing the density correcting parameter used by said density correcting means; and notifying means for notifying a user to execute a density adjustment for a copy-forgery-inhibited pattern image, when said changing means changes the density correcting parameter by an amount equal to or greater than a predetermined amount.