7a are sealed off from lubricating oil used for the bearings 5a and 5b by respective shaft seals 6a and 6b fig2 also shows schematically a plurality of screw vacuum pumps p q r connected in series in a multi stage structure forming a pump apparatus in the meantime for example the male rotor 7 is driven by an electric motor not shown through a speed change gear not shown while the female rotor 7a is rotated through a timing gear 10 with a small clearance formed between the same and the male rotor 7 a gas that is sucked in from a suction opening 8a is introduced through a suction port 8b into a groove space that is defined by the main casing 1 and the two rotors 7 and 7a and the gas then undergoes expansion and compression processes as described hereafter before being discharged from a discharge opening 9a through a discharge port 9b reference numerals 3 and 4 in fig2 denote a gear cover and a cover respectively fig1 shows the way in which the male and female rotors 7 and 7a are in mesh with each other in a view developed in the circumferential direction of the rotors in fig1 reference symbols a1 to g1 and a2 to g2 denote pairs of corresponding groove spaces of the rotors 7 and 7a a pair of groove spaces d1 and d2 define a maximum groove volume in the prior art since the trapping position of the suction port.