To reduce the number of members exposed to high temperature gas in a casing, reduce the number of parts for sliding, simplify a structure as a whole, ensure a space in a lower part of a header, and improve maintenability.

Each supporting leg 10 is erected from upper parts of horizontal supporting beams 3 installed on lower positions of both end parts of a header 7 extending in a casing, each header receiving member 11 which extends in parallel with the horizontal supporting beams 3 is fixed on upper end of each supporting leg 10, one end part of the header 7 is fixed and mounted on one side of the header receiving member 11, the other end part of the header 7 is slidably mounted on the other header receiving member 11 in its longitudinal direction, and a reinforcing material 20 for printing twist of the horizontal supporting beans 3 is bridged and installed in supporting leg 10 erecting positions and between the horizontal supporting beams 3.