PURPOSE: To reduce working period and to make quality stable, by forming into a cylindrical shape a drawn metal plate having a plurality of extension members on one surface thereof and a plurality of heat radiation fin members on the other surface thereof to constitute a tank barrel having extentions on both surfaces thereof.

CONSTITUTION: On one surface of a base 8 are arranged heat radiation fins or equi-spaced members 9, and on the other surface thereof are arranged extensions or equi-spaced members 10. The members 9, 10 have equi-spaced notches 11, 12. The base 8 is formed as a drawn and molded aluminum member 13, and the member 13 is formed into a cylindrical shape with both the members 9, 10 aligned and with abutted ends welded. Thus the working period can be shrotened and the quality can be stabilized.