VACUUM PUMP

PROBLEM TO BE SOLVED: To improve emissions performance in a high-pressure region substantially equal to an atmospheric pressure. SOLUTION: A pump chamber 3 of at least first stage from upstream end side of a vacuum pump is designed to have a capacity greater than those of pump chambers 4, 5, 6, 7 at a downstream side. Further, the pump chamber 3 includes a bypass valve mechanism 8 provided to communicate from a flow path of an exhaust port side of the pump chamber at the downstream end side in the pump stage with larger capacity to a flow path of the exhaust port side of a pump chamber at the downstream side bypassing at least one or more stages out of a plurality of pump stages of pump chambers. Then, the bypass valve mechanism 8 has a valve 8a that is opened at a high-pressure region where the pressure of a pump intake port 1 of the pump chamber 3 located at the first stage is substantially equal to atmospheric pressure, and configured to exhaust gas to a flow path at the exhaust port side of a pump chamber located at the downstream side from the pump chamber at downstream end side in a pump stage with larger capacity. COPYRIGHT: (C)2009,JPO&INPIT