1. An image inspection system, the image inspection system comprising: an image forming unit to form an image on a recording medium based on an output-target image; an image scanner to scan the image formed on the recording medium to generate a scanned image; an inspection reference image generator to generate an inspection reference image to be used for an inspection of the scanned image based on the output-target image; a computing unit to compute a difference between the inspection reference image and the scanned image; an image inspection unit to determine a defect in the scanned image based on a comparison between the difference computed by the computing unit and a threshold, wherein the image forming unit is configured to form a threshold setting image on a recording medium, the threshold setting image being prepare-able by adding an artificial defect to an original image, wherein the image scanner is configured to scan the threshold setting image to generate a scanned image including a defect, wherein the inspection reference image generator is configured to generate a normal inspection reference image based on the original image, and wherein the computing unit is configured to compute the difference between the scanned image including the defect and the normal inspection reference image; and a threshold determiner to determine the threshold based on a difference between the scanned image including the defect and the normal inspection reference image.