The level of toner color mixture in a developing device is maintained within the range not affecting the color shade of a final color image. In an image forming apparatus including four imaging units and placed along a moving intermediate transfer body, the imaging units having developing devices for developing an electrostatic latent image formed on a photoconductor to create a toner image and for collecting toner remaining on the photoconductor after transfer of the toner image, the developing device in the imaging unit performs forcible toner discharge operation when a toner mixture amount in the developing device of the imaging unit, which was computed from toner consumption of the imaging unit located on an upstream side, exceeds a predetermined value.