An exposure amount designation value calculation unit calculates an exposure amount designation value for designating the amount of exposure, which is required to image an observation target, based on an image signal. A threshold value calculation unit calculates a threshold value for comparison with the pixel value of the image signal according to the exposure amount designation value. A region detection unit detects a first region, in which the pixel value falls within a range set by the threshold value, and a second region, in which the pixel value is out of the range. An image generation unit generates an oxygen saturation image, in which the oxygen saturation is displayed differently in the first and second regions, using the image signal, the oxygen saturation, and information of the first and second regions.
