To solve problems of a defect in electrostatic charging, fogging, etc., resulting from that a more downstream imaging unit has larger re-transfer developer amount and the amount of a developer sticks and enters a contact electrostatic charging member is more because of so-called re-transfer development as to an image forming apparatus which is a tandem type and characterized in that respective imaging units are a contact electrostating charging type and a development simultaneous cleaning type.

The respective imaging units each have a mode in which a developer collected by an electrostatic charging member is discharged from the electrostatic charging member and an imaging unit which is more downstream in the moving direction of a transferred material has a longer time for the discharging mode. A means of calculating an image ratio is provided for the imaging units and the time of the discharging mode is determined according to the calculated image ratio. The time of the discharging mode is varied with imaging environment. The time of the discharging mode is determined by using the image ratio of the imaging unit itself and the image ratio of an imaging unit upstream from the imaging unit in the moving direction of the transferred material.