An image forming apparatus is provided with an image display portion such as a touch panel for displaying a preview of image data. When image data of the object to output is output with additional information added thereto or when the image data of the object to output is printed on recording paper and post processing is applied to the recording paper, the image forming apparatus combines an additional information image showing additional information or post processing with a preview image at a position corresponding to an adding position of additional information or a position at which post processing is applied in displaying the preview image by enlarging a part where the additional information image is combined to display on the touch panel.