According to an embodiment of the present invention, a ratio of a recycled toner in a first developer is detected by an auto toner sensor for recycled toner. The carrying speed of a recycled toner mixer is controlled according to a detection result. Consequently, when recycled toner density in a recycled-toner carrying chamber increases, the recycled toner mixer is rotated at high speed to discharge the recycled toner from the recycled-toner carrying chamber at high speed.