PURPOSE: To effectively prevent the destruction of a sealing section resulting from thermal expansion and the leak of coolant and the generation of a bubble following the coolant leak by providing a notched section that performs external air ventilation on a bracket arranged on the external surface of a face section and arranging a bellows assembly body that is airtightly connected to the coolant space in this notched section.

CONSTITUTION: Coolant 9 is injected from an injection port 10 and the space between both panels and the inner part of a bellows assembly body 16 are filled and sealed. When the temperature of a face section increases, inner pressure is generated in the sealed coolant 9 by the thermal expansion and carburetion of the coolant 9. This pressure presses and expands a soft bellows 18 and volume expansion components are absorbed in the bellows. The volume of the pressed and expanded components pushed out the air of a notched section 14 into atmospheric pressure through a ventilating hole 19 and as a result the internal volume is kept in stationary status. Besides, even if a bubble is generated in the sealed coolant 9, the bubble is smaller in specific gravity than liquid, the bubble floats and is collected to the bellows assembly body 16.