PURPOSE: To obtain a duct which can absorb thermal stress, by fitting part of sides of a frame body, reinforced by comparatively short braces, to the reinforcement members which consists of an outer frame of a duct by the intermediary of fixing parts, and by fitting the remainder sides to the reinforcement members by the intermediary of sliding members.

CONSTITUTION: One end of a fixing member 22 is fixed to an inner frame 21 on both sides. The other end of the fixing member 22, piercing through an outer frame 20, a heat insulating material 17, and a casing 18 which are all provided in a duct, reaches a reinforcement member 19 which is an outer frame utside of a duct, and is fixed to the reinforcement member 19. On the other hand, sliding members 23 are fitted to both upper and lower sides of the inner frame 21, being projected to the outer side, piercing through the outer frame 20, the material 17 and the casing 18, and the projected ends are sealed by metallic belows 24, respectively. The inner frame 21 is reinforced by a vertical supporting rod 25 and a horizontal supporting rod 26 arranged to the inside of the frame 21. In addition, each brace 27 is arranged among each supporting member to make the reinforcement stronger.