1. An endoscope system, comprising: an endoscope having an elongated insertional part capable of being inserted into a living body; an administering means for use in administering a fluorescent substance, which emits fluorescence with wavelengths in a first infrared spectrum to be transmitted by a living tissue more efficiently than light with wavelengths in the visible and ultraviolet spectra, and which is apt to be accumulated in a lesion, to the inside of a living body; an excitation light irradiating means for irradiating excitation light with wavelengths in a second infrared spectrum different from the first infrared spectrum to the living tissue inside the living body to which the fluorescent substance has been administered; an imaging means, incorporated in said endoscope, for cutting off excitation light and producing a fluorescence image using fluorescence with wavelengths in the first infrared spectrum emanating from the fluorescent substance; a signal processing means for processing a signal output from said imaging means and producing a video signal; and a display means for displaying an image represented by the video signal.