An endoscope system includes an endoscope with an image pickup that generates image data by capturing an image of tissue illuminated with light containing wavelength regions spaced from and not overlapping with each other. The image pickup has an RGB filter, the wavelength regions include first and second regions corresponding to a B region and to a G region of the filter. A processor calculates an index representing a molar concentration ratio of first and second biological substances contained in the tissue based on the image data including information on the first and second regions. In the first region, image data B of the tissue varies depending on the molar concentration ratio, the second region contains isosbestic points of the tissue, and, in the second region, image data G has a constant value. The processor calculates the index based on the data B and G.
