To reduce the effect of an edge in the jump-gap developing system of color image processing by setting the object of a photoconductor member related on image data that a color developed on a prescribed color plane surface is not designated in an underexposure state.

A photoconductor(OPC) 75 being adjacent to a developing sleeve 215 by giving a jump-gap distance G is arranged. By exposing the image area (first object) 235, the electric charge of -200 V is established so as to develop an image. By setting the area (second object) 230 where the image is not processed or exposed in the underexposure (partially exposed) state as for at least the present color plane surface, the electric charge of -400 V rather than the conventional electric charge of -600 V at which exposure is not executed is held by the OPC 75. Therefore, potential difference between the exposed image area 235 and the underexposure area (area which is not perfectly exposed) 230 is reduced and an electrostatic field line 250 established between them is considerably reduced. Thus, the effect of a white gap is considerably reduced or eliminated.