1. An image forming apparatus comprising: an image carrier; a developing part that develops an electrostatic latent image formed on the image carrier by using toner; a supply part that supplies a developing bias between the image carrier and the developing part, the developing bias having an AC component superimposed on a DC component; and a setting part that sets a peak-to-peak value of the AC component of the developing bias to vary between a reference value at a first frequency, the reference value being determined in advance, and a special value at a second frequency, the special value being smaller than the reference value when an image region on the image carrier passes through a developing region, the second frequency being less than the first frequency, the image region being a region on the image carrier in which an image is to be formed, and the developing region being a region in which the image carrier is opposed to the developing part.