An image inspection system for inspecting an image output on a recording medium includes an image forming unit to form a threshold setting image on the recording medium; an image scanner to scan the threshold setting image to generate a scanned image; an inspection reference image generator to generate an inspection reference image using data of the an output-target image; an image inspection unit to determine a defect in the scanned image based on a difference between the inspection reference image and the scanned image; and a threshold determiner to determine a threshold to be compared with the difference between the inspection reference image and the scanned image. The image inspection unit computes the difference between the scanned image and the inspection reference image. The threshold determiner determines a threshold to be compared with the difference between the scanned image and the inspection reference image.