PURPOSE: To enable to arrange a pair of high-stage and low-stage screw rotors in a continuous common casing, by supporting respective intermediate shaft portions of the two screw rotors by a partition wall.

CONSTITUTION: Intermediate shaft portions 21b, 24b respectively of rotor shafts 21, 24 are supported by respective shaft sealing means 27a, 27b fitted in a partition wall 26. When a rotor 20 is set into rotation by a rotor shaft 21, a rotor 23 is also rotated by means of gears 36, 37, and gas is drawn into a low-pressure stage work chamber 28 from a suction port 30 of the chamber 28. Then, the gas is compressed to an intermediate pressure through engagement of the rotor 22l and rotor 25l, and thereafter discharged to the outside from an outlet port 31. The gas thus discharged from the chamber 28 is then introduced into a high- pressure stage work chamber 39 from an inlet port 41 of the chamber 39 and discharged to the outside from an outlet port 42 after it is compressed to a predetermined discharge pressure through engagement of the rotors 22h and 25h.