A vessel suppression process is performed on RGB image data. A display of capillary vessels is suppressed by the vessel suppression process. After the vessel suppression process, tone of the RGB image data is reversed. Thereby, a suppressed-and-reversed image is produced. Even after the tone reversal, the capillary vessels do not interfere with observation of a ductal structure in the suppressed-and-reversed image, because the display of the capillary vessels is suppressed. In the suppressed-and-reversed image, the ductal structure is darker than a mucous membrane due to the tone reversal, so that the color of the ductal structure is close to that of an indigo.
