A plurality of blades are studded in a rotor disc integrated with the rotor along the circumferential direction of the rotor, a plurality of vanes are attached to a casing covering the rotor along the circumferential direction of the rotor, and an internal diaphragm disposed on rotor-side surfaces of the vanes in such a way that the internal diaphragm faces the rotor disc. The vanes and the blades adjacent to each other in the axial direction of the rotor form a turbine stage. A rotor-side cooling path is formed through the rotor disc in the axial direction of the rotor, and a diaphragm-side cooling path is formed through the internal diaphragm in the axial direction of the rotor, and a cooling medium flowing through the rotor-side cooling path diverts into the diaphragm-side cooling path and a labyrinth flow path provided between the internal diaphragm and the rotor.