PURPOSE: To improve the heat exchange efficiency and separate oil continuously by providing partition plates which are laid across the inside a drum to divide the drum into many chambers and support heat transfer tubes, and provided with a number of fine holes at the lower part.

CONSTITUTION: Liquid refrigerant containing lubricating oil is fed into an oil separator through a liquid inlet 11 at a low flow rate and flows through chambers formed by partition plates 15. On the other hand, heat source fluid is fed through an inlet 9, so that liquid refrigerant is heated via heat transfer tubes 10 and flows out from a gas outlet 14. At that time, liquid refrigerant foams over to adjacent chambers, however, the construction allows the liquid refrigerant to take enough time to contact the heat transfer tubes and evaporating until it reaches the last chambers on an outlet 12 side. Thereby, oil can be continuously separated with a high separating performance. In addition, as the partition plates 15 which divide the inside of the oil separator into many chambers support the heat transfer tubes 10, vibration is also prevented.