An image forming system is disclosed which can prevent toner stain adhered onto an image bearing member at the time of switching from one developing unit to another from adhering to an image area on an intermediate transfer member and which can thereby prevent the deterioration of image quality. Latent images formed on a photoreceptor drum are developed by developing units in a revolver developing unit, toner images thus formed are primarily transferred in an overlapped state onto an intermediate transfer belt in a primary transfer section, and the thus primarily transferred toner images are together transferred secondarily onto a transfer paper. In this image forming system, a revolving operation of the revolver developing unit is controlled so that an area on the photoreceptor drum with which a developer contained in a Bk developing unit comes into contact upon 22.5null revolution of the revolver developing unit from a home position with consequent movement of the Bk developing unit to a developing position contacts a not-to-be-transferred area on the intermediate transfer belt in the primary transfer section. By so doing, toner stain caused by the Bk developing unit which has moved to the developing position does not exert any influence on the toner images formed on the intermediate transfer belt.