To prevent poor cleaning and a failure in solid followability of an image carrier by changing the polarity of a Δ supply bias in accordance with a printing rate.SOLUTION: An image forming apparatus 100 comprises: a photoconductor drum 1 on which an electrostatic latent image based on image data input from the outside is formed; a developing roller 4 that develops the electrostatic latent image with a toner 9; a supply roller 5 that supplies the toner 9 to the developing roller 4; and an engine controller 101 including a CPU 103 that, in accordance with a printing rate based on the image data input from the outside, changes the polarity of a Δ supply bias that is the difference between a developing bias applied to the developing roller 4 and a developer supply bias applied to the supply roller 5.SELECTED DRAWING: Figure 1