PURPOSE: To provide large thermal resistance and an allowance in accuracy and make an assembly work easy by sealing the joint part of a gas duct shroud by means of a ring seal and coil spring made of ceramic.

CONSTITUTION: In the joint part A of the shroud of a gas duct 6, a ring seal 25 made of ceramic is engaged with the inner circumferential surface of a duct outer case 12, and the outer circumferential surface of the ring seal 25 is pressed into contact with the inner circumferential surface of the duct outer case 12 by the elastic force of the ring seal 25. Also, a coil spring 26 made of ceramic is made to abut on one side surface of the ring seal 25, and its energizing force presses the other side surface of the ring seal 25 into contact with the edge surface of a shroud housing 11. Thus, the airtightness of the joint part A is well maintained in both the shaft and radial directions. As stated, large thermal resistance and an allowance in accuracy can be provided and an assembly work is made easy by using the largely flexible ceramic coil spring 26 which can be manufactured and processed easily.