A light intensity adjustment system is provided which greatly decreases or makes unnecessary image compensation by the image processing device side, improves scan precision, and can reduce the scan time; and which comprises: a light irradiation device 1 that has multiple independently light intensity adjustable light irradiation units 11, and that irradiates light facing a predetermined target area A; a photographic device 2 that photographs said target area A through a lens, and outputs a target area image that is the photographed image; and a light intensity control unit 3 that controls the respective light intensities of said light irradiation units 11 so that the brightness of the various parts of the target area images that said photographic device 2 has output approaches a predetermined standard value.