Illumination light for highlighting vessels and measurement light for measuring oxygen saturation level is applied alternately to a subject. A color image sensor images the subject illuminated with the illumination light and acquires a first blue signal, a first green signal, and a first red signal. The color image sensor images the subject illuminated with the measurement light and acquires a second blue signal, a second green signal, and a second red signal. An oxygen saturation level of hemoglobin in blood is calculated using the second blue signal, the first green signal and the first red signal. The first blue signal changed in accordance with the oxygen saturation level, the first green signal, and the first red signal are assigned to respective B, G, and R channels of a display. The display displays a “high-contrast vessel image with oxygen saturation level” that varies with the oxygen saturation level.
