PURPOSE: To enable stable operation of a blow-off valve by installing a precooler between an oil free screw compressor and a check valve at the delivery side, and installing the blow-off valve at the outlet of the precooler via a blow-off cooler.

CONSTITUTION: A precooler 3 is installed between an oil free screw compressor 1 and a check valve 6 at the delivery side, and a blow-off valve 9 is installed at the outlet of the precooler via a blow-off cooler 4. Thus, during no load operation, since gas being discharged through the blow-off valve into the atomosphere is sufficiently cooled by the precooler and blow-off cooler, the blow-off valve is not damaged by heat, and the stable operation of the blow-off valve can be enabled.