PURPOSE: To reduce power consumption of a screw compressor by dividing a discharge side pipe passage into two, the first pipe passage having a check valve which feeds discharge air to an after-cooler and the second pipe passage having an ejector enabling the pressure of discharged air below the level of atmospheric pressure and a sluice valve.

CONSTITUTION: A discharge side pipe passage of a screw compressor 2 is divided into two, the first discharge side pipe which feeds discharge air to an after- cooler 5 having a check valve 4 and the second discharge side pipe passage 2c having an ejector 8 enabling the pressure of discharged air below the level of atmospheric pressure and a sluice valve 6. With this arrangement, at an unload time when a suction throttle valve 3 is fully opened while the sluice valve 6 is fully closed, an air in the first discharge pipe passage 2b is introduced into a tank 7 and is pushed out by the ejector 8, whereby the pressure before and behind the compressor 2 becomes below the level of atmospheric pressure so that it 2 does not compress the air. Consequently, the power consumption of the compressor 2 at its unload operation can be reduced.