Violet light and blue light is applied sequentially to an object. In a wavelength range (380-440 nm) of the violet light, each of reflectances of most-superficial and superficial blood vessels is lower than reflectance of middle-layer blood vessels. In a wavelength range (440-480 nm) of the blue light, the reflectance of the middle-layer blood vessels is lower than those of the most-superficial and superficial blood vessels. Brightness of mucous membrane in a normalized first signal obtained under the violet light is equivalent to that in a normalized second signal obtained under the blue light. A subtraction image is obtained by subtraction between the normalized first and second signals, and superimposed on the first signal. A special image, in which the most-superficial and superficial blood vessels correspond to falling edges and the middle-layer blood vessels correspond to rising edges and the blood vessels are distinguished from each other, is produced.
