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, an expansion process and a compression process are performed. In the expansion process, an angle between a first center line and coordinates in a specific region, which contains the first center line, is changed at an angle change rate Wnx. In the compression process, an angle between the first center line and coordinates outside the specific region is changed at an angle change rate Wny smaller than the angle change rate Wnx.