In a deep-ultraviolet tight source includes sapphire substrate, a wide band gap semiconductor layer having a wavelength smaller than 300 nm, formed on the sapphire substrate, and en electron beam source for irradiating the wide band gap semiconductor layer with an electron beam. The wide band gap semiconductor layer is configured to be irradiated with the electron beam to emit deep-ultraviolet light through the sapphire substrate. A thickness t1 of the sapphire substrate satisfies: t1≧α·E3 is an energy of the electron beam (keV); and α is 1 μm/(keV)3.