An image forming method is provided for use with an ink-jet printing device which forms a dot on a recording medium by ejecting a drop of recording liquid. In the image forming method, an image in which a ground tint is embedded in one of a foreground part and a background part of an original image is formed, the ground tint indicating that copying of the original image is inhibited. A recording liquid adhesion area of the dot per unit area in the foreground part is made different from a recording liquid adhesion area of the dot per unit area in the background part.