Abstract: A method of controlling the air output of a screw compressor, wherein the average air output of the compressor during operation is adapted to air consumption by throttling the flow of suction air or by connecting the compressor alternately to full air output capacity and idle operation capacity. In order to reduce the power consumption of the compressor, the air output is controlled first by throttling the flow of suction air and by simultaneously measuring underpressure in the suction conduit of the compressor. Thereafter the compressor is controlled so as to operate alternately at full air output capacity and idle operation capacity, and the pressure rise rate in the air-pressure network is measured when the compressor operates at full output capacity and the pressure fall rate is measured when the compressor operates at idle operation capacity.