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 a 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, the second dot image is stacked on the first dot image in at least one of the hidden text portion or the background portion, the image data forming unit places the first dot image in the hidden text portion, the image data forming unit places the second dot image in the hidden text portion and the background portion, the second dot image is stacked on the first dot image, the dots of the first dot image are magenta, dots of the second dot image in the hidden text portion are cyan and yellow, the dots of the second dot image in the hidden text portion are arranged at regular intervals, dots of the second dot image in the background portion are cyan, magenta and yellow, and the dots of the second dot image in the background portion are arranged at regular intervals.