PURPOSE: To raise the extent of back pressure by way of generating a turbulent flow in gas inside the thread groove of a static cylinder by installing a lot of limited large recesses on both outer and inner circumferential surfaces of a rotary cylinder.

CONSTITUTION: A lot of recess train belts 10 and smooth surface belts 14 are installed on the outer circumferential surface of a rotary cylinder 14 alternately in the axial direction, while other lots of recess train belts 12 and smooth surface belts 13 are installed on the inner circumferential surface alternately in the axial direction as well. When the rotary cylinder is rotated, gas inhaled out of an inlet port 19 is compressed downward by a thread groove 1 on the inner circumferential surface of a static cylinder 3 and then it advances upward along a thread groove 4 on the outer circumferential surface of a static inner cylinder 9 and exhausted out of a discharge port 20 after passing through an exhaust hole 6. In proportion as coming closet to this discharge port 20, pressure becomes high whereby a leak out of clearances 17, 18 with the rotary cylinder 14 becomes increased, but it is checked by the smooth surface belts 11, 13, while a turbulent flow is generated in these thread grooves 1, 4 owing to the recess train belts 10, 12. Therefore, each gas flow rate in these thread grooves 1, 4 is increased so that the leak is compensated and, what is more, back pressure can be heightened as well.