A correcting portion of an endoscope subjects a reference image to a magnification correction process so that the distance between a first mark and a second mark within the reference image is substantially equal to the distance between a first mark and a second mark within a comparative image. Next, the corrected reference image is subjected to a rotation correction process so that the orientation of the second mark with respect to the first mark within the reference image is substantially equal to the orientation of the second mark with respect to the first mark image within the comparison image. Further, the corrected reference image is subjected to a brightness distribution correction process so that the brightness distribution thereof becomes equal to that of the comparison image. The display state of the reference image and comparison image are made substantially equal, comparative diagnosis is facilitated, and diagnostic efficiency is improved.