The technique of the present invention enhances the workability of bonding and improves the finish in poster printing. The procedure first sets multiple areas adjacent to one another in an image expressed by master image data, where each of the multiple areas corresponds to each unit page to be printed. The procedure then enlarges each of the adjacent areas by preset dimensions (corresponding to an enlarged area in marginless printing) and sequentially extracts image data included in each enlarged area. The procedure subsequently magnifies the extracted image data by a predetermined magnification to generate each piece of page image data. The area actually printed is a little greater than the size of each sheet of printing paper. This arrangement thus enables an image part, for example, each part of a letter ‘A’, to be printed to the top, bottom, left, and right sides of each sheet of printing paper. The image part printed in each sheet of printing paper is continuous with the image part printed in an adjoining sheet of printing paper.