PURPOSE: To ensure welding and to suppress heat generation of a metallic cover so as to prevent scattering of metallic powder by making a resistance valve of a seal ring part smaller than that of the cover, which is the passage of current between electrodes for seam welding, and thereby efficiently flowing the current to a joined part between the metallic cover and a seam frame.

CONSTITUTION: In using a cover-made base material with nickel plating as a seal ring 3, the electric resistance of cover is larger than silver. However, by using a larger quantity of silver in silver-alloy brazing, the current flowing from the electrode 5 to the thickness direction of the cover 4 to the seal ring 3 to the passage C3 of the layer 2 of silver solder becomes larger than the current in other passages C1, C2. As a result, it is possible to reduce each current flowing the passages C1, C2 in performing seam welding, and to efficiently flow the current to the passage C3 related substantially to the welding. Consequently, efficient welding is made possible while reducing the welding current and time.