To provide an image forming apparatus that can convert an input image to an image composed only of a representative color faster than before.

If a selected representative color as a representative color accepted by representative color acceptance means is different from a saved representative color as a representative color stored by representative color storage means and the difference is the addition of one color ("One color addition" in S161), Voronoi diagram creation means of a multifunction peripheral executes a consecutive addition method to create a Voronoi diagram from the representative color added among the selected representative color and a saved Voronoi diagram as a Voronoi diagram stored by Voronoi diagram storage means (S164), and the representative color storage means stores the selected representative color as the saved representative color (S165).