An image processing device includes an additional image formation unit configured to form an additional image having a first predetermined density. At least a part of the additional image is 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 is 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 main image. A density of the boundary image is lower than both the first predetermined density and the second predetermined density. A synthesized image formation unit is configured to synthesize the main image, additional image and the boundary image to form a synthesized image.