An arc tube and method for forming an arc tube in which an electrode assembly including a tungsten electrode and a molybdenum foil are joined together in a partially overlapped state and pinch sealed into a pinch seal portion of the arc tube body to suppress leakage due to peeling of the molybdenum foil and prolong the life period of the arc tube. An overlapped length L1 of a tungsten electrode and a molybdenum foil is set to be 2D≦L1≦0.8W with respect to the diameter D of the tungsten electrode and the width W of the molybdenum foil. In another embodiment, the distance L2 between the molybdenum foil and a discharge space is set to be B≦L2≦0.8A with respect to the width A and the thickness B of a pinch seal portion.