A discharge tube in which a cathode with a cathode tip portion being fixed to a lead rod and an anode opposed to the cathode tip portion are encapsulated in a discharge gas atmosphere to effect arc discharge, wherein the cathode tip portion comprises: a metal substrate of an impregnated type in which a porous, refractory metal is impregnated with an electron-emissive material or a sintered type in which a refractory metal containing an electron-emissive material is sintered; and a coating of a refractory metal covering a predetermined portion in a surface of the metal substrate and having a thickness of not less than 0.02 nullm nor more than 5 nullm, wherein the metal substrate has a cusp pointed toward the anode, and wherein a tip portion of the cusp of the metal substrate is exposed without being covered by the coating.