A visible image based on light output from a region to be observed by illumination of the region to be observed with visible light, and a special image based on light output from the region to be observed by illumination of the region to be observed with special light in a wavelength band that is different from the wavelength band of the visible light are obtained. An extraction image is generated by extracting a part of image data representing the visible image. A superimposition image is generated by superimposing the generated extraction image on the special image. The generated superimposition image is displayed.