An image forming system includes circuitry. The circuitry reads an inspection target image from a printed matter. The circuitry generates a difference image between the inspection target image and a comparison target image. The circuitry performs processing on the difference image using a threshold value to perform determination as to whether a defect of the printed matter is present. The circuitry calculates a threshold value for show-through at which a show-through of the printed matter is not determined as the defect of the printed matter among multiple threshold values based on a result of the determination.