A screw vacuum pump which allows reduced power, lower inside gas temperature, and reduced discharging time. The pump has a pair of screw rotors rotatively engaged with each other in a pump casing to discharge a gas along a longitudinal direction of the pump. Each rotor has three types of helical teeth serially located in a longitudinal direction of the rotor and different from each other in theoretical displacement volume. A bypass conduit communicating with a delivery side of the pump is connected via a first check valve to a first intermediate space defined between the first helical teeth and the second helical teeth and via a second check valve to a second intermediate space defined between the second helical teeth and the third helical teeth. The gas is compressed at the third stage to half the first stage volume before a discharge port opens.