To prevent scattering of toner in an image edge part.

A conversion logical circuit 512 detects whether the printed pixel in a bit map image at every color is a pixel forming an edge or the pixel in the vicinity thereof corresponding to the reference result in a shift register referring to the pixel value of the printed pixel or the pixel group in the vicinity thereof and, when the printed pixel has the pixel forming the edge or the pixel in the vicinity thereof and a pixel value with a predetermined value or more, the pixel value corresponding to this printed pixel is converted small by a predetermined quantity to suppress the adhesion amt. of toner at an edge part in the case of lap-printing of a plurality of colors to reduce the scattering of toner to a blank part.