FIG. 25B is a plan view showing the inner side surface of the upper second housing portion 102 b shown in FIG. 25A. FIG. 25C is a plan view showing an outer side surface of the upper second housing portion 102 b shown in FIG. 25A. Referring to FIGS. 25A, 25B and 25C, the conductor layer 102 bm is made of an electrically conductive material such as magnesium or zinc, and includes a rectangular slot