A seal segment (11) includes a seal body having a plurality of laminated thin plate seal pieces (20), and a high-pressure side plate (23). The high-pressure side plate (23) includes an outer diameter side edge portion (23b) which is an outer edge portion in the radial direction (Dr) and extends in a circular are shape in the circumferential direction (Dc), an inner diameter side edge portion (23c) which is an inner edge portion in a radial direction (Dr) and extends in a circular arc shape in the circumferential direction (Dc), and a front edge portion (23d) which is an edge portion on a front side in the rotational direction (Bc). The high-pressure side plate (23) further includes a reinforcing portion (40) only in a region of the high-pressure side plate (23) on the front side in the rotational direction (Bc).