A correction function is calculated based on a reference image formed of reflected light reflected upon the irradiation of a predetermined living tissue, for which a diseased state is known, with a reference light. This correction function is employed to administer distance correction on calculated standardized values based on fluorescence images to which an offset has been added, to correct the fluctuations of the calculated standardized values caused by the distance between living tissue and the distal end of a fluorescence endoscope, thereby generating a corrected standardized fluorescence image.