An electronic endoscope (10) has normal and high-sensitivity image sensors (53, 60). In a special mode, one of the normal and high-sensitivity image sensors is selected in accordance with a reflected light amount of special illumination light. When the normal image sensor (53) is selected, the normal image sensor captures an image under irradiation with the special illumination light and outputs a second normal-sensor image. When an average pixel value of the second normal-sensor image is less than a first sensor selection threshold value, the use of the high-sensitivity image sensor (60) is started. When the high-sensitivity image sensor is selected, the high-sensitivity image sensor captures an image under irradiation with the special illumination light and outputs a high-sensitivity-sensor image. When the average pixel value of the high-sensitivity-sensor image is more than a second sensor selection threshold value, the use of the high-sensitivity image sensor is stopped.
