1. A density control method for an image forming apparatus having a plurality of image forming speeds, comprising: a first speed setting step of setting an image forming speed at which image formation is carried out by the image forming apparatus to a reference speed; a first control step of carrying out a maximum density control process at the reference speed in a manner such that image forming conditions are controlled such that a measured density of a first test image corresponding to a maximum density is equal to a predetermined value; a second control step of carrying out a density gradation control process at the reference speed to determine a conversion table for image signal conversion such that gradation characteristics of a second test image having a plurality of gradations are identical with predetermined characteristics; a second speed setting step of setting the image forming speed to a first speed other than the reference speed; and a third control step of carrying out the density gradation control process at the first speed.