1. An image forming apparatus comprising: an image data forming unit that forms an image data which includes a first dot image to be visible by copying, a second dot image to be invisible by copying, a hidden text portion and the background portion; and an image forming unit that carries out image formation by using the image data formed by the image data forming unit, wherein the first dot image has a first color, the first dot image includes dots having a dot size increased by decreasing the number of lines of dots, the second dot image has a second color, the second dot images includes dots having a dot size decreased by increasing the number of lines of dots, and the second dot image is stacked on the first dot image in at least one of a hidden text portion or a background portion.