An oxygen saturation level of hemoglobin in blood is correctly acquired without lowering a frame rate.A subject body illuminated with white light W is imaged by a color CCD to obtain signals Bs1, Gs1 and Rs1. The subject body is illuminated with blue narrow band light BN of which absorption coefficient is changed by a change in the oxygen saturation level of the hemoglobin in blood, and imaged by the color CCD to obtain signals Bs2, Gs2 and Rs2. The signal Bs2 is divided by the signal Gs1 to determine a normalized signal Bs2/Gs1. The oxygen saturation level of blood vessels of the surface of body tissue is obtained according to the normalized signal Bs2/Gs1. The oxygen saturation level is visualized in a pseudo color, to form an oxygen saturation level image.
