The present invention provides specific image adding section for adding the specific image to inputted image data includes a first counter for deciding positions of a main scanning direction and a vertical scanning direction and a second counter for specifying a form of the specific image. The second counter counts up addresses of pixels with a starting point at one corner on a matrix made up of the main scanning direction and vertical scanning direction. If the first counter counts a predetermined count value, the second counter starts. When a maximum value of the count value of the second counter is equal to or less than a count set value, an image is formed on each of the pixels. The pixel on which the image is formed is changed and the form of the specific image is also changed by changing the count set value.