An electrode (3) is joined to a wiring substrate (2). A nut box (7) is inserted in the bent portion (5) of the electrode (3) so that the nut (6) is positioned in alignment with the opening (4) of the electrode (3). A case (8) covers the wiring substrate (2). The nut box (7) and the case (8) are members separate from each other. The nut box (7) is fixed in the electrode (3) so as not to come off from the bent portion 5).