An image forming apparatus performs prints image data. A dividing section divides the image data of a print job into a plurality of sub data areas m(i) (i=1 to n). A duty computing section computes a print duty for each of the plurality of sub data areas (m(1)-m(n)) based on the number of printed dots in the print job and a total number of printable dots in a printable area. A first power supply applies a first voltage to a developing roller. A second power supply applies a second voltage to the developer supplying roller. The voltage difference between the first and second voltages is determined in accordance with the print duty.