A fixed side valve plate slides in contact with a movable side valve plate provided at a rotor of a rotary valve for supplying and discharging high-temperature high-pressure steam to and from an expansion chamber of an expander via a slide surface. The fixed side valve plate includes an expansion region having a large sliding area to which a circular steam supply passage opens, and an exhaust region having a small sliding area to which an arc-shaped steam discharge passage opens. An unsymmetrical recessed portion formed in a central portion of the slide surface and an arc-shaped notch formed at an outer peripheral portion of the expansion region, make the sliding area of the expansion region and the sliding area of the exhaust region substantially correspond to each other to make the surface pressure uniform, thereby preventing uneven abrasion of the slide surface to suppress leakage of steam.