A turbomolecular pump having a rotor provided with a plurality of rotor blades and a spacer provided with a plurality of stator blades so that gas molecules are sucked in from a suction port, compressed and discharged from an exhaust port. The pump further has a heat exchanger provided inside the suction port, the heat exchanger being connected to a refrigerator through a refrigerant pipe and a gate valve provided on the upstream side of the suction port. Gases having low molecular weights, particularly water vapor, are freeze-trapped on the heat exchanger. Thus, it is possible to efficiently exhaust gases having low molecular weights, particularly water vapor, and hence to obtain a high vacuum of good quality. In addition, it is easy to start and suspend operation of the system and possible to run it on a continuous basis.