PURPOSE: To increase pumping speed at vortex pump stages by utilizing the form of a rotor finished into a straight cylinder having blades engraved in the outer peripheral surface thereof and having no projection.
CONSTITUTION: A number of thread grooves 7 each having an angle of inclination of 20 degrees or so and a rectangular cross section and communicating recessed portions 4, 4 located at upper and lower stages and compressing air from the upper stage toward the lower stage of an exhaust passage 3 are provided in the outer peripheral portion of a rotor 2 and pumping speed is increased by utilizing both air exhausted by the thread grooves 7 and air spirally exhausted by a blade 5 extending along the circumferential direction of the exhaust passage 3.