To prevent the raising of dew point in an inner cover by sealing with a two layer structure of a spherical body material having a specific diameter as the lower layer and coarse grain seal material having a specific value of the average grain diameter.

The spherical body seal materials 2-2 having 5-30mm diameter are spread all over a seal plate 3. Conventionally used aluminas having e.g. 0.5-2mm grain diameter are spread all over on the spherical body seal material 2-2, as the coarse seal materials 2-1 at about 20mm thickness. The coarse seal materials 2-1 are filled up into the gaps among the spherical body seal materials 2-2. The inner cover 1 is laid on the coarse seal materials 2-1. The spherical body seal materials 2-2 are interposed between the seal plate 3 and the lower end surface of the inner cover 1, and the gaps are always secured. Since the lower end surface of the inner cover 1 and the spherical body seal material 2-2 are in point-contacts, the flow of atmospheric gas is not interrupted. In the case of being <5mm diameter of the spherical body seal material 2-2, there is such danger as to push up the seal material 2-2 by fluid action during the operation.