PURPOSE: To facilitate the assembly of a slidable valve element by preventing a seat ring for preventing coming-off of a valve box seat and a ring-shaped valve box seat from being projected to the movable space side even when the slidable valve element is drawn during the maintenance.

CONSTITUTION: A plurality of threaded holes 20, 21 are formed in the inner circumference of a pair of ring-shaped seal boxes 7 and in the inner circumference of seat rings 6, 6 for preventing the coming-off of a pair of valve box seat with the prescribed intervals in the circumferential direction, and a plurality of locking plates 22 provided with through holes 22A, 22B are provided. The seat rings 6, 6 for preventing the coming-off of the valve body seat are tightened to the ring-shaped seal boxes 7, 7 through a plurality of the locking plates 22 by screwing lock bolts 23, 24 into the threaded holes 20, 21 through the through holes 22A, 22B of the lock plates 22, and the seat rings 6, 6 for preventing the coming-off of the valve box seat and the ring-shaped valve box seat 5A are prevented from being projected to the movable space side of the slidable valve disc even when the slidable valve element 1 is drawn.