PURPOSE: To permit effective liquefaction in a easy-to-operate and low-cost facility by having at least one type of C2-C4 saturated hydrocarbon added to pressurized evaporated-gas and then heat-exchanged with liquefied natural gas from a storage facility to liquefy the evaporated gas.

CONSTITUTION: BOG which is given out of a LNG storage tank 1 is pressurized by a BOG compressor 2 to add thereto at least one type of saturated hydrocarbon, such as ethane, propane or butane, led through a heavy-gravity lead line 9 and sent to a heat exchanger 3. On the other hand, LNG which is pressurized up by a LNG pump 6 through the LNG storage tank 1 is led into the heat exchanger 3. Then, heavy-gravity added BOG is cooled and liquefied for storage in a drum 4 and, after returned to the tank 1 or pressurized up by a pump 7, jointed to the LNG which is extracted through the heat exchanger 3, to be sent to consumers through a sending gas pipe system via a LNG carbureter 5. As a result, it is possible to perform effective liquefaction in a easy-to-operate and low-cost facility.