In a special mode for imaging an oxygen saturation level of blood, an internal body portion is imaged under irradiation with special illumination light. A light amount evaluation section judges based on an obtained image whether or not a reflected light amount of the special illumination light is adequate for calculating the oxygen saturation level. When the reflected light amount is judged to be adequate, a normal image sensor captures an image under irradiation with the special illumination light. When the reflected light amount is judged to be low, a high-sensitivity image sensor is used. In using the high-sensitivity image sensor, a binning process is applied to an image signal in accordance with the reflected light amount of the special illumination light, in order to further sensitize the image signal.
