A first special image is produced by combining an overall bright normal light image with a blue-enhanced image in which blood vessels and their shapes are enhanced, and displayed on a display device. A suspected lesion such as a spot is detected in the first special image. When the suspected lesion is found, an oxygen saturation level of the suspected lesion is obtained. When the oxygen saturation level of the suspected lesion is in a hypoxic condition within a predetermined range, an oxygen saturation image representing the oxygen saturation level is displayed instead of the first special image on the display device.
