It is an object of this invention to provide an image processing apparatus which, when nonmagnetic mono-component development is performed, prevents the formation of white gaps in pattern boundaries between different colors and thereby prevents degradation in the image quality of the formed image. To achieve this object, in one embodiment of the invention, the density value VD(n) of the nth pixel on a certain line is applied to the A input terminal of a selector 207, and the density value VD(n-1) of the (n-1)th pixel is applied to the B input terminal of the selector 207. A subtracter 203 calculates the increase of VD(n) from VD(n-1). If a comparator 204 determines that the increase is a predetermined value C1 or less, a selection signal in the selector 207 is so set that the terminal B (the value of the immediately preceding pixel) is selected.