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There is provided a mercury-free arc tube for a discharge lamp unit. The mercury-free arc tube includes a plurality of electrodes and a sealed chamber including a metal halide and a starting rare gas enclosed in the sealed chamber. A clearness index value P 2 ·W/Á is equal to or greater than about 800, where Á denotes a density (mg/cm 3 ) of the enclosed metal halide, P denotes a pressure (atmospheres) of the enclosed starting rare gas, and W denotes a maximum input power (watts) input to the sealed chamber through the electrodes; in order to improve the luminous-intensity-rise characteristic after turning on the arc tube.