PURPOSE: To prevent residual toner from being mixed in developer after cleaning the photosensitive drum of a color image forming device.

CONSTITUTION: Electrostatic charging potential VH impressed on a photosensitive drum 30 is set within the range of -500 to -900V, and the DC component VDC of developing bias is set within the range of -400 to -800V so as to be superposed. Exposure potential VL for exposing the area of the residual toner (d) is set within the range of 0 to 3O0V, and the laser beam from a laser write system unit forming an image, for example, is used also for an exposure means. The toner whose charge amount is within the range of -10 to -30μC/g is used, and the residual toner (d) to which laser-exposure is performed, adheres to the photosensitive drum 30 whose potential is lowered with a specified adhesive force.