15 has an upper portion thereof extending from the housing 12 a multiplicity of rotor blades 16 are fixedly secured to the peripheral surface of an upper section of the extending portion of the rotary shaft 15 a rotor 17 is fixedly mounted between the rotor blade assembly and the housing 12 so as to cover or surround the same the rotor 17 is comprised of an upper end wall 17b and an annular portion 17c connected thereto a helical groove 17a of a trapezoidal cross section is formed in the outer peripheral surface of the annular portion 17c a stator 18 forms an outer case of the molecular pump 6 and a slight gap is maintained between the stator 18 and the outer peripheral surface of the rotor 17 inside an upper portion of the stator 18 stator blades 19 are fixedly secured at positions overlapping the rotor blades 16 as electric current is caused to pass through the coils of the stator 13 from the electric power supply device not shown a rotary component comprised of the rotary shaft 15 motor rotor 14 rotor blades 16 and rotor 17 is rotated at high speed so that gas molecules introduced through the suction port 10 are mechanically blown off by the rotor blades 16 and the trapezoidal helical groove 17a and are discharged through the exhaust port 7 to thereby produce a pumping action however if the pressure on the exhaust side is high the molecular pump 6 cannot be operated because extremely high power is required the molecular pump 6 can be operated if the pressure at the exhaust port 7 is brought to a level equal to or less than.