PURPOSE: To enable improvement of efficiency and high quality production by forming the flange part of a cup into a step shape while shortening the flange part of an inner for being housed inside the cup flange part.

CONSTITUTION: Four places of the flange part 2b of a cup 2 are made step- shaped while forming the flange part 3b of an inner 3 short in four places for being housed inside the flange part 2b of the cup 2. Since, the flange part 3b of the inner 3 is housed inside the flange part 2 of the cup 2 in this way, junction with a convergence cup 4 is completed at the side of the flange part 2b of the cup 2. Accordingly, inspite of possible positional shifting of the tip of the flange part 2b to the focal point of laser light 5, a tolerance range is widened thus reducing inferiority such as peeling-off of welding.