A seal assembly which is arranged to be inserted into a space, the space being defined by a pair of concave portions provided on side surfaces of transition piece outlet flanges of transition pieces which are provided in each of a plurality of combustors that are adjacent to each other in the mounted state in a gas turbine so that the side surfaces face each other, wherein the concave portions each have a first side surface which is located on a downstream side of the concave portion with respect to combustion gas flowing through the transition pieces, a second side surface which is located on an upstream side and faces the first side surface, and a bottom surface, the seal assembly comprising: a longitudinal direction that corresponds to an insertion direction of the seal assembly into the space, a seal body comprising a seal plate which is arranged such that, in the inserted state in the space, it faces the first side surface of each of the pair of concave portions and which has a seal portion contacting the first side surfaces in the mounted state, and a spring portion arranged to press the seal plate toward the downstream side, the spring portion comprising a first spring plate and a second spring plate which is adjacent to the first spring plate, wherein the first spring plate and the second spring plate include a seal plate contacting portion at which it is joined to a surface on the upstream side of the seal plate by welding, wherein each spring plate (51A,51B) extends from the seal plate contacting portion to the upstream side so that a part of each spring plate (51A,51B)contacts the second side surface of each of the pair of concave portions, and wherein the first spring plate passes through a through-hole which is formed in the second spring plate, characterized in that the seal body comprises side plates which extend from opposite side edges transverse to the longitudinal direction of the seal plate toward the upstream side along the bottom surface of the concave portions. a longitudinal direction that corresponds to an insertion direction of the seal assembly into the space, a seal body comprising a seal plate which is arranged such that, in the inserted state in the space, it faces the first side surface of each of the pair of concave portions and which has a seal portion contacting the first side surfaces in the mounted state, and a spring portion arranged to press the seal plate toward the downstream side, the spring portion comprising a first spring plate and a second spring plate which is adjacent to the first spring plate, wherein the first spring plate and the second spring plate include a seal plate contacting portion at which it is joined to a surface on the upstream side of the seal plate by welding, wherein each spring plate (51A,51B) extends from the seal plate contacting portion to the upstream side so that a part of each spring plate (51A,51B)contacts the second side surface of each of the pair of concave portions, and wherein the first spring plate passes through a through-hole which is formed in the second spring plate, a first spring plate and a second spring plate which is adjacent to the first spring plate, wherein the first spring plate and the second spring plate include a seal plate contacting portion at which it is joined to a surface on the upstream side of the seal plate by welding, wherein each spring plate (51A,51B) extends from the seal plate contacting portion to the upstream side so that a part of each spring plate (51A,51B)contacts the second side surface of each of the pair of concave portions, and wherein the first spring plate passes through a through-hole which is formed in the second spring plate, characterized in that the seal body comprises side plates which extend from opposite side edges transverse to the longitudinal direction of the seal plate toward the upstream side along the bottom surface of the concave portions.