One or more through-holes communicating with one or more of the plurality of screw grooves are formed at the cylindrical stator, a total of circumferential dimensions of the one or more through-holes formed at the cylindrical stator is set at equal to or greater than a circumferential dimension of an outer peripheral surface region of the cylindrical stator facing the second suction port, and a gas path through which inflow gas through the second suction port is guided to a screw groove is provided, the one or more through-holes penetrating through the screw groove and the screw groove being apart from the region facing the second suction port.