An internal cylinder which has the opening section from which combustion gas is blown out is inserted into the tail cylinder which becomes a flow path of the combustion gas, and is engaged with the tail cylinder, while securing a clearance between the edge of the opening section and the internal wall surface of the tail cylinder. Air compressed with a compressor is jetted from the clearance along the internal wall surface of the tail cylinder, and a film of cooling air is formed on the internal wall surface of the tail cylinder.