PURPOSE: To prevent deterioration of the result factor of a vaporizer and to enable execution of unmanned operation by a method wherein a pressure in an oil return passage at a stage after the passage of a refrigerant through an expansion valve is set to a value higher than a pressure in a suction piping connected to the suction side of a refrigerator.

CONSTITUTION: Lubricating oil gathering in an oil reservoir 6 of a vaporizer 4 can be returned to the liquid receiver 3 side in a way that a refrigerating cycle is periodically switched and a pressure on the vaporizer 4 side is increased to a value higher than that of the liquid receiver 3 side. When a refrigerator is under refrigerating operation, lubricating oil present in an oil reservoir part 23 of the liquid receiver 3 can be constantly returned to the suction piping 38 side of a freezer 1 through an expansion valve 25 togetherwith a refrigerant. However, since a pressure on the suction piping 38 side on the refrigerator side is set to a value lower than a pressure in an oil return passage 48 running back from the liquid receiver 3, lubricating oil can be returned to the refrigerator through suction pipings 38 and 39. Since a refrigerant containing lubricating oil is expanded and cooled by means of the expansion valve 25, temperature in the suction piping 39 is lowered, the temperature of a refrigerant on the a freezer delivery piping 31 side is also not increased and lubricating oil is prevented from deterioration.