PURPOSE: To prevent the dropping of a spiral fin thus to improve the heat radiating characteristic and to improve the reliability considerably, by securing the spiral fin firmly to a heat generating member.

CONSTITUTION: The bent section 5a of a spiral fin 5 is contacted tightly to the outercircumference of a tubular anode 2 through a metal band 6 and secured by a caulking section 5c, thereby the heat transmission from the anode 2 will be improved considerably to such extent as approximately 300W500°C when compared with the conventional temperature rise of 250W300°C, and the heat efficiency is improved considerably while since it is fastened by a metal band 6 having small thermal expansion factor, the fitting strength sufficient for the repetation of the external machanical force and the heat cycle thus to eliminate the falling-out of the spiral fin 5.