1. An image processing device comprising: an image generator which generates an image to be formed on one or more sheets each including at least one gluing margin and a normal region other than the gluing margin, so that a whole image of one page is obtained when the gluing margins of the one or more sheets are laid on top of each other; and an image processor which performs image processing on the image generated by the image generator, wherein the image processor performs the image processing on the image differently depending on whether the image is to be formed on the gluing margin or on the normal region in such a way that an image on the gluing margins laid on top of each other and an image on the normal region have a same density.