A flange 37b of a cap case 37 is provided with protrusions 37c that, before being swaging, project upward from the flange 37b. When the flange 37b is swaged along an upper surface of the cap 3, the protrusion 37c projects along the upper surface of the cap 3 from the flange 37b toward a center of the cap 3. The cap case 37 is welded to the cap 3 by friction stir welding at a welding portion 37d located at a substantially center of the protrusion 37c. The protrusions 37c are formed approximately in a letter-T shape that is symmetric left and right, and width W2 of a connecting portion 37r in a cap circumferential direction is smaller than the maximum width W1. The rigidity of the connecting portion 37r against deformation is reduced by forming a portion with small width W1 at the connecting portion 37r.