A method for disassembling/assembling a gas turbine including a seal plate disposed on a first side of a rotor disc in an axial direction of the rotor disc, and a seal plate restraint part for restricting movement of the seal plate in a radial direction of the rotor disc. The method includes a seal-plate-restraint-state switching step of operating the seal plate restraint part from a second side in the axial direction via a clearance between a platform of a blade and a region of an outer peripheral surface of the rotor disc except a blade groove for receiving the blade to switch between a seal plate non-restraint state where the seal plate restraint part does not restrict movement of the seal plate in the radial direction and a seal plate restraint state where at least a part of the seal plate restraint part protrudes in the axial direction.