PURPOSE: To prevent the rotation stop of a screw due to the sticking of a foreign object or the like by forming at least either spiral screws or the inner wall face of a casing facing them with a rugged surface.

CONSTITUTION: Rotors 3a, 3b are constituted in a pair, one rotor 3a has a valley-shaped spiral screw 21a, and the other rotor 3b has a ridge-shaped spiral screw 21b. When suction is performed from the processing space with a vacuum pump, a foreign object in the processing space may be mingled in the air A to infiltrate into a rotor space 4. If the sticking and accumulation of the foreign object proceeds to some degree, the foreign object is cut from the inside face 4a of the rotor space by the rugged shape of a groove section 22a provided on the peripheral face of the screw 21a, and the cut chips 30a are discharged to the outside of the rotor space 4 through an exhaust port 6 together with the air A.