1. An image forming apparatus, comprising: an added image detecting section that detects added image data contained in image data by analyzing input image data to be printed; an added image determination section that, when added image data is detected by the added image detecting section, determines whether the detected added image data corresponds to any post-processing on a post-processing apparatus which applies post-processing to a printing medium which has been subjected to printing; an image forming section that forms a printed image with added image data removed from image data to be printed if the added image determination section determines that the added image data corresponds to a post-processing; and a post-processing controller that instructs the post-processing apparatus to apply post-processing for the added image data if the added image determination section determines that the added image data corresponds to post-processing.