A mercury-free arc tube for a discharge lamp unit including a closed glass bulb in which main light emitting substances of NaI, ScI3, and ScBr3 and buffer substances of InI and ZnI2 are enclosed together with Xe gas, and wherein an amount of enclosed ScBr3 is in a range of about 5 wt % to about 24 wt % relative to a total weight of the substances enclosed in the closed glass bulb. The mercury-free arc tube may also include a shroud glass tube, which is surrounds the closed glass bulb and a pair of electrodes, which are disposed to be opposed to each other in the closed glass bulb.