A rotary valve for switching supply passages, and discharge passages of high-temperature and high-pressure steam for a rotor rotatably supported at a casing is constructed by making a fixed side valve plate and a movable side valve plate abut to each other on a slide surface. A pressure chamber into which the high-temperature and high-pressure steam is introduced from the supply passage is opened to a mating surface of a valve body portion with the fixed side valve plate. The fixed side valve plate supported to float is pressed against the movable side valve plate with a pressing load generating in the pressure chamber to bring it into close contact with the slide surface. Leakage of the high-temperature and high-pressure steam from the pressure chamber is prevented by placing a V-packing inside the pressure chamber.