Excitation light is inputted to an optical element with uniformly dispersed inorganic fluorescent nanoparticles whose radius is controlled. Fluorescence is emitted from the inorganic fluorescent nanoparticles through photoexcitation and outputted from one end of the optical element. By controlling the radius of the inorganic fluorescent nanoparticles dispersed in the optical element, the wavelength range and spectral width of the light outputted from the optical element are controlled.