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 superimposed, wherein the image generator generates an image on only one of an upper one and a lower one of the gluing margins of the one or more sheets; 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, wherein superimposed, and an image on the normal region are observed to have a same density when the images are observed by a viewer, the image processor performs the image processing in block units, the image generator extracts, from the image, one or more objects to be located on the gluing margin and adjusts a width of the gluing margin to be an integral multiple of a width of a block of the block units in which the image processing is performed on any of the extracted objects, and the image generator adjusts the width of the gluing margin with reference to an object having an attribute for which gradation is the most important among the objects to be located on the gluing margin.