A plate spring 6 having a hook-shaped cross section from the lower part is installed in the neighborhood of one end “b” of an impingement-cooling plate 4. One end “c” on the lower side is fixed to a rib 1d by welding, while the other end “b” on the upper side is free, which makes it closely contact with the neighborhood of the other end “b” of the impingement-cooling plate 4 by the elastic force thereof. This state makes it possible to seal a gap formed between the impingement-cooling plate 4 and a transition piece 1 on the side of a rib 1d, preventing thermal stress generated in the rib 1d, for example, from affecting the impingement-cooling plate 4.