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.