PURPOSE: To hold the respective developing devices in an appropriate concentration state and to satisfactorily maintain the quality of a copy by applying alternately a toner concentration control of an AIDC system to plural developing devices, in case of executing continuously a simultaneous two-color copy.

CONSTITUTION: In a continuous copy operation, for instance, in a first (first sheet) copy operation, when a toner concentration control of an AIDC (detecting directly by an optical sensor) system is executed to a black developing device 5, in a second (second sheet) copy operation, a toner concentration control of the AIDC system is executed to a color developing device 4. Thereafter, the toner concentration control of the AIDC system is applied alternately to the developing devices 4, 5. In this regard, in the first copy operation, the toner concentration control of the AIDC system is applied to the developing device in which an image area to be generated is larger. In such a way, the toner concentration of the respective developing devices 4, 5 is held appropriately, and the generation of a picture quality failure is prevented.