3 denotes a flow guide portion 4 denotes a flow liner 5 denotes an inne rsurface heat insulating portion 6 denotes an outer surface heat insulating portion 7 denotes a bolt 1 a and 2 a denote flanges and 5 a denotes a segment the present embodiment is the above described embodiment as the best mode of the present invention in which the outer surface heat insulating portion 6 has been added to the outer surface of the outer tube portion 2 explanations overlapping those offered in the aforementioned best mode are omitted in a plant which has a gas turbine housed in an enclosure not shown and is placed under a controlled atmosphere for protection of a fire alarm and measuring instruments heat dissipation from an exhaust diffuser may be a cause of a temperature rise in the enclosure and this is improper however because of dimensional restrictions for example an adequate thickness of the inner surface heat insulating portion 5 may be impossible to ensure in this case a temperature rise within the enclosure maybe induced according to the gas turbine concerned with the present embodiment mentioned above however if a sufficient thickness of the inner surface heat insulating portion 5 is not obtained because of dimensional restrictions etc the outer surface heat insulating portion 6 is added to the outer surface of the outer tube portion 2 by so doing heat dissipation to the enclosure can be prevented whereby the temperature rise within.