A steam turbine (10) is provided with a double-structure comprising an inner casing (20) and an outer casing (21). A turbine rotor (22), in which plural stages of moving blades (24) are circumferentially implanted, is operatively disposed in inner casing (20). A diaphragm outer ring (25) and a diaphragm inner ring are disposed along the circumferential direction in inner casing (20). Stationary blades (27) are circumferentially provided between diaphragm outer ring (25) and the diaphragm inner ring, so that diaphragm outer ring (25), the diaphragm inner ring and stationary blades (27) form a stage of stationary blades. The stages of the stationary blades are arranged alternately with the stages of moving blades (24) in the axial direction of turbine rotor (22). A cooling medium passage (40) for passing a cooling medium CM which is supplied through a supply pipe (45) is formed between inner casing (20) and diaphragm outer ring (25).