First RGB image signals are inputted. A first B/G ratio and a first G/R ratio are calculated. The first B/G ratio and the first G/R ratio are converted into a second B/G ratio and a second G/R ratio, respectively, through a color information conversion process. Owing to the color information conversion process, a difference between first and second observation areas in a second signal ratio space formed by the second B/G ratio and the second G/R ratio is greater than a difference between the first and second observation areas in a first signal ratio space formed by the first B/G ratio and the first G/R ratio, and a difference between the first and third observation areas in the second signal ratio space is greater than a difference between the first and third observation areas in the first signal ratio space.
