PURPOSE: To improve the cooling function and to reduce the cost of attaching man- power and the added material, by employing a cooling fin formed spirally and tightly over the outer-circumference of anode tube.

CONSTITUTION: A cooling fin 20 formed spirally and tightly over the outer-circumference of anode tube 1a with its circular outline of plane viewed along the axial direction of the anode tube 1a is adhered through an adhesive 8 to the anode tube 1a. The adhesive 8 preferably provide good thermal conductivity at the joint of them. The cooling fin 20 may be fitted in a groove made in the anode tube 1a.