A first signal ratio (−log(B/G)) between a B image signal and a G image signal is calculated. A second signal ratio (−log(G/R)) between the G image signal and an R image signal is calculated. A difference between first and second signal ratios in a first area and first and second signal ratios in a second area is increased to enhance a color difference between normal mucosa and an abnormal region (an atrophic mucosal region and a deep blood vessel region). An image after enhancing the color difference is displayed on a monitor.
