To ensure energy saving by disposing an air-cooled condenser with its head difference from an evaporator and a compressor, and returning a liquid refrigerant from the air-cooled condenser into the evaporator by the head difference to construct a refrigeration cycle without operating the compressor.

High temperature high pressure refrigerant gas compressed by a centrifugal compressor 3 is introduced into an air-cooled condenser 1 where it is cooled with a fan 1A into a liquid refrigerant. The low temperature low pressure liquid refrigerant emanating from the air-cooled condenser is changed to a low temperature low pressure liquid refrigerant upon passing through an expansion valve 7 and is collected in the evaporator 2 where it is changed to refrigerant gas through a heat transfer pipe 2A, and the refrigerant gas is compressed with the centrifugal compressor 3 and is again changed to high temperature high pressure refrigerant gas. Then, operation of a turbo refrigerator in an intermediate interval is performed by interrupting a driving source 4 of the centrifugal compressor 3 and fully opening an opening/closing valve 6. The refrigerant gas evaporated in the evaporator 2 is cooled into a liquid refrigerant in the air cooling condenser 1 and flowing into the evaporator 2 with the aid of a head difference, and thereafter the operation is done in the same cycle as a refrigeration cycle due to compressor driving.