Generating an image of a molar concentration ratio between a first and second biological substance, an absorption spectrum of the first and second biological substance within a predetermined wavelength range including a first, second, third, and fourth isosbestic point, acquiring first imaging data of the tissue using light extracted from white light using a first filter to collectively selectively extract light in a first wavelength range demarcated by the first and second isosbestic point, light in a second wavelength range demarcated by the second and third isosbestic point, and light in a third wavelength range demarcated by the third and fourth isosbestic point； acquiring second imaging data by taking an image of the tissue using light from the white light using a second optical filter to selectively extract light in the second wavelength range； and generating the distribution image on the basis of the first and second imaging data.
