To provide a vent valve easy to operate and construct and inexpensive to manufacture for preventing the whirling-up of particles in a vacuum chamber and the degradation of throughput.

A valve element 21 is provided in an internal space 12 of a casing 11, and the valve element 21 is energized toward its fully opened position by a spring 31. An air cylinder 51 is provided for displacing the valve element 21 toward its closed position against the spring 31. When the valve element 21 is at the closed position and a through-hole 24 is closed, an inlet passage 16 is cut off from an output passage 17, and when the through-hole 24 is opened, it is communicated therewith. Except at the closed position, the inlet passage 16 is communicated with the outlet passage 17 via the through-hole 24 and a tapered groove 13 in an inner face 11a of the casing 11. The cross section area of the tapered groove 13 is increased with an increase in the displacement of the valve element 21 from the closed position.