FIG. 20A is a plan view of the cover 8 showing other example having a plurality of radiating pins 8d on the top face thereof, and FIG. 20B is a side view of the cover 8 in FIG. 20A. The radiating pins 8d in this embodiment is formed by decreasing the thickness of the cover 8 making the total thickness of the cover 8 equal to that of the cover 8 in FIG. 8.