Provided are: an image processing device that can emphasize changes in color of a mucus membrane or the like that can occur during atrophy of the stomach due to atrophic gastritis and a method for operating an endoscope system. A first signal ratio (-log(B/G)) is calculated between a B image signal and a G image signal, and a second signal ratio (-log(G/R)) is calculated between a G image signal and an R image signal. A process is performed that magnifies the differences between the first and second signal ratios in a first area and between the first and second signal ratios in a second area, thus emphasizing the color difference between normal mucus membrane and an abnormal region (atrophied mucus membrane region and deep blood vessel region). A post-color-emphasizing image is displayed at a monitor.
