There is provided, in a light source having at least an excitation light source, a rare-earth doped, fluoride optical waveguide, a multiplexing means and an optical fiber for output, an ASE light source characterized in that an ASE light that has a wavelength shorter than an excitation light and that has been generated by an up-conversion process generated in an inside of the rare-earth doped, optical waveguide is outputted to outside via at least one long-wavelength cut-off device provided inside or outside of an apparatus.