An additional image is formed on a recording medium by filling a predetermined area including a punched portion or a stapled portion, in addition to an image which is to be formed originally, such as a text document. The additional image is formed with white developer or transparent developer. Since the thickness of the recording medium is increased by the white developer or transparent developer forming the additional image portion, the region around the punched portion or the stapled portion is reinforced and the recording medium is kept from being broken easily. When the image and the additional image are superposed, the superposition of the image and the additional image is avoided by either reducing the image or changing the position of the image to be formed.