1. An image processing device, comprising: an additional image formation unit configured to form an additional image having a first predetermined density, at least a part of the additional image being overlaid with a main image having a second predetermined density, the main image being formed on an image formation medium; a boundary image formation unit configured to form a boundary image which is formed in an area defined in the vicinity of an overlaid area where the additional image is overlaid with the additional image, a density of the boundary image being lower than both the first predetermined density and the second predetermined density; and a synthesized image formation unit configured to synthesize the main image, the additional image and the boundary image to form a synthesized image.