PURPOSE: To reduce energy loss, and to achieve smooth gas exhaust by forming screw parts in multiple stages on a screw rotor, and by providing a relief valve on an inter-stage position of the screw rotor of a casing, while a gas introducing port on a screw rotor position on the exhaust side.

CONSTITUTION: Screw rotors 7, 8, 9 are coaxially installed along shaft at a certain interval, and a screw part of a screw rotor is formed in each stage so as to form the transfer capacity of the screw rotor on the exhaust side smaller than that on the suction side. On the outer surface of a casing 2 on the side of each gap, relief valves 10, 11 are mounted, so as to prevent the pressure of the gas exceeding which gas is in the gap between the respective screw rotors 7, 8, 9. A gas introduction port 12 is connected to the side surface of the casing 2 on the position of the third stage screw rotor, through which the gas in a form of air pressure is supplied in the casing 2 in the vicinity of the third screw rotor 9, and a certain flow of gas stream is formed in the casing 2 and is discharged from an exhaust port 4 at the same time.