A main stop valve includes: a hydraulic cylinder; a piston arranged in a hydraulic cylinder; a valve rod connected to the piston; and a valve body connected to the valve rod and configured to block a steam passage. The main stop valve further includes: a pilot cylinder having a communication port in fluid communication with the hydraulic cylinder; a rod connected in parallel with the valve rod and configured to be inserted from one end of the pilot cylinder; a sliding ring provided on the rod; and a pilot sleeve configured to be slidably fitted into an inner surface of the pilot cylinder and slidably fitted into an outer peripheral surface of the sliding ring in a section between the one end of the pilot cylinder and the communication port.