PURPOSE: To provide a through conduit type gate valve in which leakage can be stopped by increasing the pressure on the seat surface when a leakage exceeding a specified amount is caused.

CONSTITUTION: When a leakage of a fluid 35 is caused by the deformation of the seat surface or the reduction of the pressure on the seat surface due to the obsolescence or the like of an elastic body 13 that has been arranged between a valve casing 1 and a movable seat 10, the valve element 20 is moved more than a predetermined stroke in its shut direction, so that a gear is engagedly turned through a rack. The outside seat part 12 is engagedly turned with regard to the inside seat part 11, and is moved so as to approach the valve element 20 side, and the outside seat part 12 is pushed against the valve element 20 side to increase the seat surface pressure, so that the leakage of the fluid 35 is stopped. The changeover of the valve from the closed state to an open valve state is carried out by screwedly turning the outside seat part 12 in the reverse direction so that the valve is moved so as to be separated from the valve element 20 side. Thus, the seat surface pressure is reduced, and the movement of the valve element 20 is lightly carried out.