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 is performed such that coordinates corresponding to a second observation area are moved to a reference area containing the origin point while coordinates corresponding to first and third observation areas are maintained unchanged. A second process is performed to move the coordinates corresponding to the first observation area and the coordinates corresponding to the third observation area away from each other.
