RGB image signals are inputted. B/G ratio is calculated based on B image signal and G image signal. G/R ratio is calculated based on the G image signal and R image signal. In a feature space formed by the B/G ratio and the G/R ratio, a third process for increasing a difference in saturation between coordinates in a first observation area and coordinates in a second observation area is performed. In the first observation area, coordinates corresponding to a portion infected with H. pylori are distributed. In the second observation area, coordinates corresponding to a portion in which eradication of the H. pylori infection has been successful are distributed.