According to the invention, a vacuum pump is disclosed comprising a screw-rotor pump having a compression section (8) and an expansion section (7), and wherein a discharge (10) from the compression section communicates with at least one ejector (1) for discharge of compressed gas through the ejector, and wherein the expansion section (7) is connectable via a first valve means (5), to a drive-gas source (P) for operating the screw,-rotor pump and the ejector in parallel. Also, a method for providing sub-pressure to an industrial process is disclosed wherein at least one ejector (1) is used initially to reduce the pressure to a predetermined lower level, from where the pressure is further reduced by means of a screw-rotor pump (7, 8) that is arranged to operate through, and in parallel with the ejector.