PURPOSE: To provide a compact freezing system by a method wherein oil and refrigerant are heat exchanged through an oil cooler so as to cool the oil and the refrigerant is returned back to an intermediate region of a screw compressor.

CONSTITUTION: Oil is separated by an oil separator 2 for a refrigerant gas discharged from a screw compressor 1, the refrigerant is flowed to an air-cooled condensor 3 and the oil is flowed through an oil pipe 8 to an oil cooler 6. The refrigerant bypassed at an outlet of the condensor 3 passes through a bypassing valve 7, flows to the oil cooler 6 and the refrigerant is heat exchanged with the oil at the oil cooler 6. The heat exchanged oil and the refrigerant enter from each of inlet ports 21 and 22 to the screw compressor 1. The oil enters the inlet port 21 to lubricate a bearing or the like. The refrigerant enters from the inlet port 22 after closing the rotor or when a two-stage compressor, the refrigerant may enter between an intermediate stage or a low pressure closing and an intermediate pressure stage. A temperature of oil at the outlet of the oil cooler 6 is defined by adjusting a degree of opening of a bypassing valve 7, and this degree of opening is adjusted by sensing a temperature of the refrigerant gas at an outlet port of the oil cooler 6. Accordingly, it is not influenced by a condition of application of either an air-cooled condensor 3 or an evaporator 5 and thus an oil of stable temperature is supplied to the bearing or the like.