To provide an exhaust valve can shorten exhaust time without stirring up foreign particles.

To solve two contradictive requirements of exhausting gas without stirring up foreign particles and shortening the exhaust time, a slow exhaust valve is obtained that has a large exhaust resistance so as not to stir up foreign particles when the valve starts to open, reduces the exhaust resistance continuously with its operation stroke, and has small exhaust resistance so as to be able to utilize an exhaust pump effectively when the valve is fully opened.