Abstract: A fluorescence detecting system detects auto fluorescence emitted from an intrinsic pigment in a part of an organism to be observed. An excitation light source projects onto the part to be observed excitation light in the wavelength range which can excite the intrinsic pigment of the organism to emit auto fluorescence. A first fluorescence detector extracts from the auto fluorescence emitted from the pigment a whole auto fluorescence component in a visible region having a predetermined wavelength range including a first relatively short wavelength range and a relatively long wavelength range. A second fluorescence detector extracts an auto fluorescence component in a second relatively short wavelength range in the visible region from the auto fluorescence. A divider carries out a division between the auto fluorescence components respectively extracted by the first and second fluorescence detector.