A seal segment (11) is provided with: a retainer (21); a seal body (13) comprising a thin plate seal piece (20); a high pressure-side side plate (23); a low pressure-side side plate; and a block body(50) that comprises a retainer simulation section (51) extending further in the circumferential direction (Dc) from the edge of the retainer (21) in the circumferential direction (Dc) and a seal bodysimulation section (53) extending inward in the radial direction (Dr) from the retainer simulation section (51). The high pressure-side side plate (23) and the low pressure-side side plate cover at least part of the seal simulation section (53) in the block body (50).