It is intended to effectively cool a dummy ring and a rotor disposed on the inner side of the dummy ring of a single-flow turbine and to suppress a decrease in thermal efficiency by preventing main steam from leaking to the dummy ring side. A cooling steam supply pipe 32 is provided in the dummy ring 26 of the single-flow turbine 10A and extraction steam of a boiler at 570° C. or below is supplied to a clearance c between the dummy ring 26 and the turbine rotor 12 as cooling steam S4. The cooling steam S4 has lower temperature and higher pressure than leak steam S2 which is a portion of the main steam S1 leaking to the dummy ring 26 side. By supplying the cooling steam S4, the leak steam S2 is prevented from entering the dummy ring 26 side and the dummy ring 26, a welding part w and a second rotor part 12b with low heat resistance that are disposed on the inner side of the dummy ring 26 can be cooled.