First RGB image signals are inputted. Color difference signals Cr and Cb are calculated from the first RGB image signals. In a feature space formed by the color difference signals Cr and Cb, a first process and a second process are performed. In the first process, coordinates which correspond to the first, second, and third observation areas are moved in a parallel manner such that the coordinates which correspond to the second observation area are moved to a reference area that contains the origin point. In the second process, the coordinates which correspond to the first observation area and the coordinates which correspond to the third observation area are moved away from each other.