PURPOSE: To protect a low vacuum piping system from back flow into a process chamber by making pressurized air flow from an inflow port of a solenoid vent valve to its outflow port when applying a current to the solenoid vent valve, and communicating the outflow port with the atmosphere or a vent line and closing the inflow port when cutting off electricity.

CONSTITUTION: In the state of a current not flowing to a solenoid valve 29, an exhaust port 31 communicates with a vent port 32 to ventilate. When a current flows to the solenoid valve 29, an inflow port 30 communicates directly with the exhaust port 31, and air is not ventilated through the vent port 32. The air passes through an exhaust line and moves through an inflow port 37 of a quick exhaust valve 36. A movable flapper member in the valve 36 works to seal an exhaust port 39, and the air passes through the valve 36 from the inflow port 37 and flows outside from an outflow port 38. A low vacuum piping system is therefore protected from an accident and breaking causing back flow into a process chamber, and breaking of a turbo pump is prevented.