By installing a cylinder 53 having a bell-mouth construction to the downstream side of a combustor basket 2a, compressed air flowing between the combustor basket 2a and an external cylinder 2c is throttled. In addition, by having a cross section of the downstream-side end surface of the cylinder 53 shaped in a semicircle and providing a back surface wall 2d having a curvature being equivalent to a curvature of the semicircular curved surface, compressed air flow making 180 degrees turn can be made uniform and free from disturbance.