An image capturing system, comprising: a light source device (208) that emits illumination light containing wavelength regions； an image pickup device (108) having an RGB filter； and an image processing unit (220) 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. The second index is defined as a value obtained by deviding the image data G by the image data R. The image processing unit is configured to calculate a third index representing a degree of possibility of malignant tumor of the living tissue based on the first index and the second index.
