FIG. 22B is a plan view showing the inner side surface of the upper second housing portion 102 b shown in FIG. 22A. FIG. 22C is a plan view showing an outer side surface of the upper second housing portion 102 b shown in FIG. 22A. Referring to FIGS. 22A, 22B and 22C, an electrical conductor layer 102 bm made of an electrically conductive material such as magnesium or zinc is formed on the inner si