PURPOSE: To make the captioned pump compact by installing the thread grooves of a thread groove type compression pump which is provided on a suction port side in parallel in a radial direction and simultaneously introducing a gaseous body in a vacuum chamber into each of said thread grooves.

CONSTITUTION: A gaseous body which is sucked into a casing 1 is simultaneously introduced into thread grooves 17aW17c. The gaseous bodies in the thread groove 17a and the outside thread groove 17b are introduced from a bottom end opening part 1a while the gaseous body in the inside thread groove 17c is introduced from an inside bottom end, into the upper main flow passage 12 of a vortex type compression pump 15 through the suction passage 11b of its upside housing 11. Thereby, an installing space can be reduced as far as possible, making the captioned pump compact.