In a special observation mode, an oxygen saturation frame period, a normal frame period, and a vessel pattern frame period are repeatedly performed. A brightness detector (75) detects the brightness of a latest frame image of an oxygen saturation video image, being a key video image. The intensity of light to be applied in the next oxygen saturation frame period is determined from the detected brightness. From the determined light intensity and a light intensity ratio among frames, the intensity of light to be applied in the next normal frame period and the next vessel pattern frame period is calculated. The exposure time of the next oxygen saturation frame period is determined from the detected brightness. From the determined exposure time and an exposure time ratio among frames, the exposure time of the next normal frame period and the next vessel pattern frame period is determined.
