To separate oil contained in refrigerant gas efficiently through an oil separator by disposing the oil separator at a position in a refrigerant circuit corresponding to a position where refrigerant gas delivered from a compressor takes superheated steam state close to wet steam on Mollier diagram.

A heat exchanger 6 and a cooling fan 7 are disposed between a compressor 1 and an oil separator 5. Depending on the condensation temperature of refrigerant detected by a temperature detector 8 and the refrigerant temperature at the inlet of the oil separator 5 detected by a temperature detector 10, r.p.m. of the cooling fan 7 is varied to regulate the heat exchanging quantity of a heat exchanger thus 6 making constant the difference between the condensation temperature of refrigerant and the refrigerant temperature at the inlet of the oil separator 5 at all times. When the refrigerant gas temperature is lowered, current velocity of the refrigerant gas is decreased and oil separation efficiency is enhanced. When the refrigerant gas is overcooled, the refrigerant gas takes wet steam state to cause a trouble. The refrigerant is fed to the oil separator 5 at a position where refrigerant takes superheated steam state close to wet steam state thus enhancing oil separation efficiency.