PURPOSE: To lowly retain the temp. of a nonmetal bellows and to reduce thermal stress generated in a rib structure and to assure long life by forming strength ribs having a specific intermediate plate material and an installing rib having low thermal conduction.

CONSTITUTION: A rib structure for supporting a nonmetal bellows 7 is provided with a radiation mechanism 4 and strength ribs 1, 2 serving as strength members provided with an intermediate plate material 3 having a thermal shielding function for retaining low temp. by shielding the nonmetal bellows 7 from parts resulting in high heat. In addition, provided is an installing rib 6 serving as a functional member in which its one end is connected to the strength rib 1 and its other end is fixed to the nonmetal bellows 7 and the quantity of heat passing through the members is reduced to reduce heat transfer. Thus the nonmetal bellows 7 is shielded from the surroundings of the strength ribs 1, 2 liable to comparatively raise temp., resulting in reducing temp. elevation in the nonmetal bellows 7. Heat transfer to the nonmetal bellows 7 is also reduced to retain the nonmetal bellows 7 to durable temp. or below.