A color image sensor captures a reflected image of narrowband light having a wavelength range in which an extinction coefficient varies with a change in an oxygen saturation level of hemoglobin in blood. Thereby a first blue signal, a first green signal, and a first red signal are obtained. The color image sensor captures a reflected image of white light. Thereby a second blue signal, a second green signal, and a second red signal are obtained. Only an oxygen saturation level, out of two or more types of biological functional information including a blood volume and the oxygen saturation level, is obtained based on the first blue signal, the second green signal, and the second red signal. The oxygen saturation level is visualized to produce an oxygen saturation image.
