A vacuum valve is provided with opening pressure setting means that sets a set value of a fluid pressure in a primary side passage beyond which communication is opened and forcible opening means that forcibly opens communication regardless of the fluid pressure in the primary side passage. The opening pressure setting means includes a spring receiver fixed to a stem, a compression coil spring having a lower end supported by the spring receiver, and an adjustment screw, movable in the axial direction, that presses the upper end of the compression coil spring. The forcible opening means includes a piston inserted onto the stem 5 under the spring receiver so as to be movable upward and downward and piston driving means that moves the piston and the spring receiver upward integrally by moving the piston upward using compressed air.
