1. An image processor for combining a computer-readable coded image into a print image of a document including a document data main body and at least one associated data piece, comprising: a print image generator that generates a print image of a document including an image of at least a part of the associated data; a coded image generator that generates a coded image to be combined into the print image; and a controller that determines whether or not the generated coded image can be combined into the image portion of the associated data by comparing a size of the generated coded image with a size of an image portion of the associated data, and combines the generated coded image into the image portion of the associated data on the generated print image, if it is determined that combining is possible.