1. A printing system comprising: an addition unit that creates additional image data having a specific attribute, to be added to original image data input from an external source, and that adds the additional image data to the original image data so as to create document image data; an image forming unit that forms, on a recording medium, on the basis of the document image data, a document image including an original image associated with the original image data and an additional image associated with the additional image data; an image reader that reads the document image formed on the recording medium by the image forming unit so as to create read image data; a difference image creator that creates difference image data by calculating a difference between the original image data and the read image data; a canceling unit that corrects, on the basis of the specific attribute of the additional image data, the difference image data by canceling a difference generated due to the addition of the additional image data so as to create corrected difference image data; and an inspection unit that performs inspection to find a defect of the original image included in the document image formed on the recording medium by the image forming unit, on the basis of the corrected difference image data.