PROBLEM TO BE SOLVED: To efficiently liquefy and reduce the boil-off gas(BOG) extracted from a storage tank with a simple structure. SOLUTION: The BOG in a liquefied natural gas(LNG) storage tank 10 is extracted and compressed by a BOG compressor 16. A cold storage medium 34 solidified near the liquefying temperature of the BOG is stored in a cold storage container 32, and a prescribed circulating liquid is circulated through a heat transfer pipe 36 immersed in the cold storage medium 34. The circulating liquid, the compressed BOG, and the LNG fed from the LNG storage tank 10 are concurrently heat-exchanged by a heat exchanger 18.