RGB image signals are inputted. A B/G ratio is calculated from the B and G image signals. A G/R ratio is calculated from the G and R image signals. First, second, and third areas are located in a feature space formed by the B/G and G/R ratios. An equal angular magnification process is performed on an angle in a region R1x including a first reference line passing through the second area. An angle expansion process or an angle compression process is performed on an angle in a region R1y located outside the region R1x. An equal radial-coordinate magnification process is performed on a radial coordinate in a region R2x, which includes a second reference line passing through the second area and intersecting the first reference line. A radial-coordinate expansion process or a radial-coordinate compression process is performed on a radial coordinate in a region R2y located outside the region R2x.
