In a seal assembly which is inserted into a space, the space being defined by a pair of concave portions provided on side surfaces of transition piece outlet flanges, including: a seal body in which a seal portion, which contacts each of side surfaces of a downstream side of combustion gas flowing through the transition piece of side surfaces which face each other in each of the pair of concave portions, is formed, and an elastic body which presses the seal body toward the downstream side, wherein the seal body includes a seal plate and a side plate, and the elastic body includes a seal plate contacting portion and an elastic portion.