When an imaging-device (39) obtains an image of an observation-region, a constant brightness of the image is maintained, and a blur of the observation-region is reduced. When the light-amount of the illumination-light illuminated the observation-region is less than or equal to first-threshold-value, electronic-shutter-speed is controlled at first-constant-speed, and the light-amount of the illumination-light is controlled based on the light-amount of the illuminated illumination-light. When the light-amount of the illuminated illumination-light is greater than first-threshold-value and less than or equal to second-threshold-value, electronic-shutter-speed is controlled at a speed based on the light-amount of the illuminated illumination-light, and the light-amount of the illumination-light is controlled at a predetermined constant light-amount. When the light-amount of the illuminated illumination-light is greater than second-threshold-value, electronic-shutter-speed is controlled at second-constant-speed, and the light-amount of the illumination-light is controlled based on the light-amount of the illuminated illumination-light.
