PURPOSE: To effect easy capacity control of a screw compressor by driving a turbo-compressor which is disposed at the suction side of the screw compressor by means of a variable speed motor and controlling revolution of the turbo- compressor on the basis of the quantity of used discharge air.

CONSTITUTION: An intercooler 3 and a turbo-compressor 4 are positioned in a suction passage of a screw compressor 1 which is directly connected to an inducation motor 2, and an impeller shaft of the turbo-compressor 4 is connected to a variable speed motor 6 with a speed-increasing gear 5 interposing therebetween. A speed controller 7 is connected to the variable speed motor 6. A pressure pick-up 8 which is disposed in a discharge line 9 of the screw compressor 1 is connected to the speed controller 7. The pressure pick-up 8 detects a capacity in the form of a pressure, on the basis of which, the speed controller 7 controls revolutions of the variable speed motor 6. Capacity control of the screw compressor 1 is hence achieved by controlling revolutions of the turbo- compressor 4.