PURPOSE: To reduce the friction coefficient between seats and to reduce the operating force to open a valve element by providing an opening to eject the high pressure liquid onto at least either of joined surfaces on the valve element side or on the yoke side in a gate where the valve element slides along the valve yoke.

CONSTITUTION: In gates used for the clean water supply system, the sewage system, and the river water system, a valve element seat 4 is brought into pressed contact with a valve yoke seat 5 due to the hydrostatic load in the direction of the arrow. When a gate is opened in this condition, a pump is first driven, the high pressure air or the high pressure water is fed through a hose 10 to a passage 7 provided in a valve element 1 and a passage 9 provided in a valve yoke 6, and an air cushion or a high pressure water cushion is formed between the valve element seat 4 and the valve yoke seat 5. This constitution reduces the friction coefficient between the respective seats 4, 5 close to 0, and under this condition, opens the gate by driving an opening/closing device 3 and pulling the valve element 1 upward. This arrangement allows easy opening/closing operation of the gate, and downsizes the driving device 3.