PURPOSE: To provide a valve usable as a cutoff valve of reliability by actuating an actuator piston independently from an inlet channel inner pressure.

CONSTITUTION: This valve 1 is provided with a movable valve disc 21 pressed to a related valve seat 7 by pressure in a fluid inlet channel 6 when it is at a closure position, it is connected to the valve disc 21 on the opposite side to the fluid inlet channel 6, and it is also provided with a valve shaft 12 having an actuator piston 13 positioned in a pressure chamber 16. When the pressure chamber 16 is pressurized, the valve 1 is opened, and fluid in the fluid inlet channel 6 goes through the valve seat 7 to be discharged to an outlet channel 9. The valve shaft 12 has a shaft piston 10 positioned in a sealed condition in a valve guide 11. The shaft piston 10 has a larger diameter Dp than the minimum diameter DS of the valve seat 7. Pressure at the outlet channel 9 is thus applied to the valve shaft 12 by force in a valve closing direction.