An image forming apparatus for printing image data of print subject after converting the image data to print image data dependent on an image forming device, the apparatus including a control section which converts the image data to first data in a color space independent from the image forming device by referring to a first conversion table, converts the print image data to second data in the color space by referring to a second conversion table, calculates a color difference specified by a difference between the first data and the second data, and causes a display section to display the calculated color difference.