PURPOSE: To provide a double-stage screw pump which restrains rise of intermediate pressure in an intermediate passage and reduce driving torque.

CONSTITUTION: A vacuum vessel 21 is connected to a front-stage suction port 27 of a front-stage pump 22, and a discharge port 28 is connected to a rear-stage suction port 29 of a rear-stage pump 24 by an intermediate passage 23. At a communication passage 31 for making the intermediate passage 23 communication to the suction port of the front-stage pump, an opening/closing valve 33 is provided. An accelerating gear 26 is provided at an output shaft of a driving machine 25, and it is connected to male rotors 7A and 7B. A gas in the vacuum vessel 21 is discharged from the rear-stage discharge port 30 through the front- stage pump, the intermediate passage 23 and the rear-stage pump, and inside of the vacuum vessel 21 is maintained at a high vacuum. A pressure switch 32 is provided at the suction port of the front-stage pump, and when a pressure more than a predetermined pressure is detected, the opening/closing valve 33 is opened, and when a pressure less than the predetermined pressure is detected, it is closed.