To prevent the occurrence of cracks in a joint area between a light emission part and a thin tube in a discharging initial stage, and to prevent occurrence of color change during lighting.

In an arc tube comprising ceramic thin tubes with an electrode mount inserted therein, and a ceramic light emitting part 1 with the thin tubes 2 jointed to both ends, the thin tubes and the light emitting part have joint regions; and the diameter of the electrode mount in the joint region is set smaller than that on the end side of the arc tube of the electrode mount.