1. An image display apparatus comprising: an image obtainment unit that obtains 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; an extraction image generation unit that generates an extraction image by extracting at least one outline included in the visible image; a superimposition processing unit that generates a superimposition image by superimposing the extraction image generated by the extraction image generation unit on the special image; and a display unit that displays the superimposition image generated by the superimposition processing unit.