An image forming apparatus is provided that makes it possible to visually identify whether or not additional information is added to image data to be output in a screen in which a preview is displayed easily. The image forming apparatus is provided with an image display portion such as a touch panel for displaying a preview of image data and is capable of outputting such as printing the image data with additional information added thereto. When image data to be output is output with additional information added thereto, a digital multi-functional peripheral synthesizes an existence image showing an existence of the additional information, instead of the additional information, with a preview image to display on the image display portion in displaying the preview image.