A steam turbine of an opposed-current single-casing type has a high pressure turbine part and an intermediate-pressure turbine part housed in a single casing. A dummy ring partitions the high-pressure turbine part and the intermediate-pressure part, and a cooling steam supply path and a cooling steam discharge path are formed in the dummy ring in the radial direction. Extraction steam or discharge steam of the high-pressure turbine part, whose temperature is not less than that of the steam having passed through a first-stage stator blade, is supplied to the cooling steam supply path. The cooling steam is fed throughout the clearance to improve the cooling effect of the dummy ring and a turbine rotor. The cooling steam is then discharged through a cooling steam discharge path to a discharge steam pipe which supplies the steam to a subsequent steam turbine.