PURPOSE: To adjust the amount of intake air and the amount of oil automatically during low revolution and control the number of revolutions to 0% ratio of the amount of the used air in a screw compressor for controlling the number of revolutions.

CONSTITUTION: A suction throttle valve 2 driven by a pulse motor 18A to control number of revolutions to no load condition and an oiling throttle valve 1 driven by a pulse motor 18B in an oiling system are provided in an inverter driven screw compressor having a motor 4, an inverter 7 and a compressor body 3, etc. A PID device 21 for comparing and calculating detected current of the motor 4 with its set current and a control device 22 for receiving a frequency signal from the inverter 7 and controlling the pulse motor according to the signal from the PID device 21 are also mounted in the compressor. Thus, inverter driving to 0% ratio of amount of air used (no load) can be carried out as capacity control of the screw compressor, so that a large energy saving effect can be obtained. A piping can be simplified by applying an electric system to control of a suction throttle valve so that manufacturing manhours can be reduced.