An image capturing system, comprising: a light source device that emits illumination light containing wavelength regions; an image pickup device having an RGB filter; and an image processing unit to calculate a first index representing a molar concentration ratio of a first biological substance and a second biological substance contained in the living tissue, wherein the wavelength regions comprise: a first wavelength region corresponding to a B filter; and a second wavelength region corresponding to a G filter, wherein: in the first wavelength region, a value of image data B varies depending on the molar concentration ratio; the second wavelength region contains a plurality of isosbestic points of the living tissue, and, in the second wavelength region, a value of image data G takes a constant value without depending on the molar concentration ratio.