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 relative to the rotor disc in a radial direction of the rotor disc includes a seal-plate-restraint-state switching step of operating the seal plate restraint part from a second side in the axial direction 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 toward the second side in the axial direction from the seal plate and thereby restricts movement of the seal plate in the radial direction.