To make it possible to generate an estimated spectral image having intended color tone even from image information taken using different illumination light and thereby increase the accuracy of a diagnosis. An estimated spectral image generating section generates a first estimated spectral image for a first endoscope observation image according to a prescribed wavelength set. On the other hand, a wavelength set setting means sets an approximate wavelength set that approximately produces a spectrum of illumination light emitted from a first light source and the estimated spectral image generating section generates a second estimated spectral image for a second endoscope observation image according to the thus-set approximate wavelength set.
